Cut the tomatoes into chunks and put them in a colander to drain juice, reserve liquid since you may need it later.
Put oil, lime juice, and serrano peppers and a couple of jalapenos in a blender, and chop up very fine. If it's too thick add a little juice from tomatoes.
Chop onions in food processor, not too fine, chop tomatoes in processor, not too fine, fine chop garlic in processor, add cilantro to processor ( I add a couple of mild banana peppers for taste) I also use Italian parsley in place of cilantro, If it's too hot for your taste add more tomatoes .
I puree the peppers, so when you eat it, you can taste the flavor of the vegetables, then you get the taste of the hot peppers (I used some small about one inch long peppers much stronger than jalapenos.)