This is perfect to eat with makeshift Mexican dishes but the leftover sauce will also be a good condiment for other meals.
1/4 raw brown onion
1 handful of washed spinach leaves (baby or not)
Juice of 1/2 a lemon
1/4-1/2 can of tomatoes (you can use fresh ones but you will have to add some water too)
Literally put all the ingredients into a container and use a whizz (those hand-blender things) to mix them all up. Serve it straight away but it will keep for a few days in the fridge.