1 28 oz can whole tomatoes
1/2 yellow onion, cut into two wedges
2 cloves garlic
2 serrano peppers
3/4 cup cilantro (or more -- we like lots of cilantro)
1 tsp cumin
1/2 tsp chili powder
1 tsp salt
pinch of sugar
2 Tb lime juice
Set the broiler to High. Line a baking sheet with foil, and spray with cooking spray or drizzle with oil.
Place the onion wedges, unpeeled garlic cloves, and serrano peppers onto the baking sheet and under the broiler.
Let broil for about 5 minutes, until things are beginning to char. Turn the onions, garlic, and peppers such that the blackened sides are facing downward, and broil for another 5 minutes.
Remove from the oven and place the peppers in a plastic sandwich bag to steam (this helps with peeling off the skin). Once cool, peel the garlic cloves and roughly chop. Clean away the blackened skin from the peppers, chop off and discard the stem, and remove the seeds (or leave the seeds in for a spicier salsa); then roughly chop the peppers.
Add tomatoes (with the juice), onion, garlic, serranos, cilantro, spices, and lime juice into the bowl of a large food processor. Pulse a few times and then puree until smooth. Taste and adjust seasonings to taste.
Note that salsa is best when made at least a day in advance. Store it in the refrigerator, and it should last about a week.