Finely chop the onion and place it in a small bowl. Using a grater, grate the garlic into the bowl (or use a garlic press or very finely chop it with a knife.)
Cut the lime in half and juice half of it. You should have about 2 tablespoons. Add the salt and the lime juice to the garlic and onions, stir, and set aside for a few minutes while chopping the other ingredients.
Slice the pepper in half and taste for heat. If the pepper isn’t too hot, finely mince it. If it is very hot, remove the seeds and membrane from one or both halves, and then mince. You should have about 1 1/2 to 2 tablespoons of pepper. Add to to the onions.
Chop the tomatoes into medium chunks and add to the bowl. Roughly chop the cilantro and add to the bowl. Stir gently to combine.
Taste the pico de gallo. If needed, add more salt, more cilantro, or more lime juice from the remaining half of the lime.
If possible, cover and let rest for a few hours in the fridge or on the counter to blend the flavors. Best if used the day it is made.
Serve with chips, bread, or as a topping on your favorite mexican-inspired dishes.