If you have a comal, or stove-top grille, count your blessings and use it to grill the vegetables until blackened (for the peppers and onion) or darkened and mushy (for the tomatoes). Use your nose and eyes, if it stops looking good, you've gone too far. If you have no comal (like me), use a big skillet over high heat with 1/2 t. salt in it to roast the veggies.
When all is as it should be, toss ingredients into a blender and pulse to chop until uniform. LOOK OUT, the top may blow off from the released heat, so loosen the little thing-y that comes out of the top, or don't plug it on too tightly. When blended, add salt to taste.
If you had bad luck, or bad produce management, and the stuff is too thick, add some water (bottled is best).
Toss the now thick and gorgeous goo into a storage container (glass is best). Pop it into the fridge and let it cool off for 30 min. Before serving.
By the way, for all you non-Angelenos, Salsa means Sauce and Cocida means Cooked, and it is considered to be a staple of most Mexican tables.
Disfruta-la