Don't be afraid of the chipotle or jalapeño chillies - there are easily sourced ingredients or substitutes, and it's not too hot a salsa, honest!
This is a healthy salsa I put together to go with some fish tacos - it would also be great with either fish (such as sea bass, cod or tuna) or grilled chicken, in a wrap, as a dip, or even in a sandwich with some cream cheese!
Serves four to eight, 24 to 49 calories per portion (depending on whether between four or eight). We made it for four people (to go inside eight tacos) and had half leftover when we'd finished.
4 vine (or plum) tomatoes (425g before skinning/de-coring) 
1 tsp neutral tasting oil (e.g. sunflower) 
1 small red onion, cut into small dice (80g) 
3 garlic cloves, finely chopped 
1 fresh jalapeño chilli (or 1 large green or red chilli will be fine), deseeded and finely chopped 
1 tbsp chipotle chilli sauce, or more/less to taste (Heinz do a version, which has no artificial nasties in it, and is widely available in the UK in supermarkets) 
Salt and freshly ground black pepper to taste 
Handful of fresh coriander, chopped (with a few leaves reserved to garnish) 
Pre-heat to gas mark 6 / 200C / 400F. Cut the tomatoes in half, through their equator (not from top to bottom). Put in a dish, or on a non-stick tray, cut side up, and roast in the oven for 20 minutes, then finish under a hot grill (broiler) for a few minutes, until they have a little colour on the cut side.
Leave to cool for a few minutes before handling, turn cut side down over kitchen towel to drain any excess juices; then skin them, cut in half again and cut out the core, and chop them up then add to a bowl.
Meanwhile, heat the oil in a good, non-stick pan, and sautee the onion for a few minutes until softened and just starting to colour. Add the garlic and chilli, and cook for a further minute or so, until fragrant, then add to the bowl with the tomatoes.
Add the chipotle chilli sauce (to taste) and the fresh coriander, give it a good stir, then season to taste and serve garnished with the reserved coriander leaves. This will keep in the fridge for a few days, and you can prepare the day before if you wish and leave the flavours to develop.