Roast the onion and peppers (bell, poblano, and Serrano) on a hot grill, and grill until blackened and charred. Or place them on a baking sheet and broil until the peels are black, turning once or twice to roast all the sides of the vegetables.
Take the peel off the onion. Coarsely chop the onion.
Remove the seeds and membranes from the center of the bell pepper. Cut the stem end of the spicy peppers and remove the seeds and membranes for a mild salsa; leave the seeds and membranes in if you want a hotter salsa.
Put all the ingredients in a food processor or blender and blend until desired chunkiness. If the salsa is too thick, thin with a little water, about 1 tablespoon at a time until desired consistency is reached.