I make this in a food processor, and it's really fast and easy. If you aren't using one, the prep time goes up, but the result is just as yummy. I'll give directions both ways.
With a food processor:
1) Let tomatoes drain while you chop garlic and juice 1/2 lime. Slice serrano pepper into rings, being careful not to touch anything but the outside of the pepper (wash hands anyway after slicing it, to avoid skin irritation). I don't like de-seeding hot peppers, since it's so much easier to just coarsely chop & toss them into the processor. Half of a serrano does make a pretty spicy salsa (delightfully so, to me!); so if you're not sure you love spicy, either de-seed the serrano, use 1/4 instead of 1/2 of the serrano pepper, or substitute bell pepper.
2) Quarter or coarsely chop onion (& bell pepper, if you're using that substitution). Add onion, pepper, & garlic to food processor container, and pulse about 5 times, scraping down sides between pulses. Add 3 of the whole tomatoes, and repeat pulsing (scraping down sides as needed) until the serrano pepper appears to be finely diced, with no large pieces of pepper visible (if using bell pepper, this step is less important... just whirl it a bit til the onion is chopped). Then add all the rest of the ingredients, and pulse 2-3 more times until it's blended but still chunky... don't overdo it! It just takes a very few pulses; more makes a texture more like picante sauce than salsa. When you've got it blended but still chunky, transfer to non-reactive bowl and refrigerate.
For preparation by hand:
Drain tomatoes, reserving liquid for another use. Mince garlic (it needs to be more finely chopped than if it were going into the processor), and chop other veggies coarsely or finely, depending on whether you want chunkier or smoother salsa. Wear gloves if possible while dicing the serrano. Otherwise it's hard to finely dice hot peppers by hand without burning your fingertips (at least it's hard for me)... you can also try using a fork to hold the pepper still while you slice it in strips, around the tines; then use the back of the fork to hold the strips down while you dice them. Combine chopped tomato, onion, pepper, & cilantro in a large non-reactive bowl. Add lime juice, cumin, salt, and oil, and mix well.
3) Whichever preparation method you use, chill salsa at least an hour before serving, for best flavor. (I usually can't manage to follow my own advice here, it's so tasty I feel I have to eat it right away...but it really is even better after the flavors blend for awhile!)
Enjoy!
Source of recipe: I came up with this recipe for when I don't have fresh tomatoes, but still want fresh salsa.