I only do this in the summer when I can get good tomatoes (i.e., from my own garden, but from a farmer's market will do as well).
coarse chop a couple of cups of tomatoes. This time of year you have such great tomatoes, go for it.
fresh cilantro. I honestly cannot taste dry cilantro at all (taste buds aren't what they used to be). This has to be "to taste" because some don't like it.
Juice of one lime
One small or 1/2 large onion, finely chopped
Here is another place you have to use your judgement. Peppers vary a lot in heat. One pepper from the same plant might be hotter than hades and the next one not so much. So chop a fresh pepper and add it as you taste.
There's a big bowl of this in my fridge.