Combine the serrano pepper, onion, salt, and lime in a large bowl and allow to marinate for 10 minutes.
Place the tomatoes in a colander or strainer and sprinkle with a pinch or two of salt. Let rest for 10 minutes, while the onion mixture is marinating. This is done to drain excess water from tomatoes and avoid a watery salsa.
Add the tomatoes to the onion mixture, along with the garlic and cilantro. Mix to combine, then season to taste with salt and pepper. Toss again, and adjust seasonings accordingly, adding more salt or lime juice as needed. Allow to rest for 5-10 minutes to allow the flavors to meld.