One of the foundations of Thai curries, a good green curry paste is essential and not at all difficult to make.
This is the base for the most classic of all Thai curries, used in curries with coconut milk/cream, garnished with kaffir lime leaves, chillies and Thai sweet/holy basil. Green curries are generally thinner, hotter and more pungent than red curries.
Makes approximately 160g in total (including a tbsp water) therefore 10 servings of 16g each (approximately one tbsp) at 14 calories per serving.
This will keep in the fridge for 2-4 weeks - although rather than leave it for that long and/or have it go off, I recommend giving it a couple of days for the flavour to develop in the fridge, then freezing it in one-serving portions (you could use an ice cube tray) so you've always got fresh-flavoured curry paste to hand. It will keep in the freezer for months.
Tip - it's not a bad idea to wear food-safe rubber/plastic gloves to de-seed and chop the chillies - or else be VERY careful where you put your fingers for a day or so after making this!!
2 tbsp coriander roots, scraped and chopped (or stalks – save the leaves - if you don’t have roots, use about 10 stalks per root, or about 4-5 tbsp chopped stalks - if you feel like you won't use all the leaves, you can blend them with a splash of water and freeze in an ice cube tray to add to Thai and Indian curries etc.) 
25g green birdseye chillies (about 15 chillies), halved lengthwise, de-seeded and finely chopped 
½ tbsp salt
25g lemongrass, peeled (the first layer, second if tough and dry) and finely sliced (soak in warm water after slicing for 20-30 minutes, if they’re tough/dry) 
20g galangal (or ginger), peeled and finely sliced 
5g kaffir lime zest (or normal, if you can’t get kaffir, or use kaffir lime leaves instead), finely chopped (or grated) 
3 red shallots (or 50g red onion if you can’t get shallots), finely sliced (or chopped if onion) 
25g garlic peeled and finely sliced, or put through a garlic crusher 
2 tsp shrimp paste (vegetarians or vegans could omit and substitute an extra tsp salt to avoid impairing the flavour, or use yellow bean paste, or a medium miso paste instead - beware of wheat/gluten if using yellow bean or miso pastes - Clearspring organic white miso is GF) 
The roasted and/or ground spices
1 tsp coriander seeds, roasted and ground 
½ tsp cumin seeds, roasted and ground 
½ tsp white peppercorns (use other peppercorns, if you don’t have white, or omit), ground 
You can make the curry paste with a pestle and mortar if you want to be traditional (and are up for some heavy duty pounding of ingredients!), or with a food blender (or processor) if you want to save time and effort.
If using a pestle and mortar, you add individual ingredients separately, starting with the hardest and driest, to the softest and wettest, reducing each ingredient to a pulp before adding the next. You will get a superior flavour and texture to making it in a food processor, but it is a laborious and time-consuming, messy task!
So, rather than being put off making your own curry pastes (which are nothing like the insipid versions bought from supermarkets in jars, you might prefer to at least start out by using your food blender (or processor).
It's easiest to grind the spices in an electric spice grinder / coffee grinder, if you have one, before blitzing the wet ingredients - I use this one frequently for small quantities of dry spices, which only cost me a few pounds and does the job perfectly - I've had it for a few years now.
If you use a bender, you may need to blend smaller amounts at a time, and may even need to add a little water (only a tbsp or so) to ensure a smooth blend of the ingredients. Don’t be tempted to add oil though, as it will emulsify. If you do add water to part of a batch, you can then squeeze it out, (maybe using a fine sieve), to add to the next batch/ingredient, then squeeze out and add again, to concentrate the flavour rather than just adding more and more water each time. I use a Thermomix, and grind the dry spices first on Speed 10 for a few seconds then add the wet ingredients starting with the hardest (lemon grass), blitzing and scraping as I go.
A further option, is to pass the ingredients through a mincer a few times, which won’t require added water, using a fine mincer plate at the end.
Store unused paste in the fridge for 2-4 weeks, or preferably freeze after a couple of days in portions.
Notes and tips
Please note that I have used bird's eye chillies in this, which are very hot if you’re not familiar with them, although I have reduced the amount a little. However, an authentic green curry is HOT, (unlike some of the insipid versions found in supermarkets in plastic containers) and if you don’t like hot food, you may wish to substitute the bird's eye chillies for some large green deseeded chillies (25g of flesh, or less) instead. Maybe just one or two if you want to be cautious – remember, you can always add extra heat, with sliced chillies added to the broth, or sprinkled on top (raw, or fried), but you can’t take it away!
Making the paste yourself may seem like a bit of a faff, but it’s well worth it for the flavour, so if you like it, next time double or quadruple it, as there are so many different green curries (even here!), it's great to have a supply in the freezer.
N.B. Blue Dragon Thai Green Curry paste is 108 calories for 100g, suggests ¼ can for a two person serving, which is 35.6g per person, and therefore 38 calories per person, so a big saving on calories as well as being a whole lot tastier and cheaper in the long run! If you're going to use bought green curry paste rather than making your own, I'd recommend the 'Thai Taste' brand.