Roast the onion and pepper on a hot 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, about 15 minutes.
Take the peel off the onion. Coarsely chop the onion. Cut the stem end of the spicy pepper 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 and pulse 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.