Cut the stems out of the tomatoes, using your knife to remove most of the white core. Slice the tops from the jalapenos or serrano chiles. If you want a milder salsa, you can slice the chiles in half and remove the seeds and ribs, or some o the seeds and ribs if you want a more medium level of heat.
If you want to grill the vegetables, preheat a grill to medium and place the tomatoes and peppers and garlic on a large piece of foil, pull up the edges of the foil to prevent them from spilling off the sides, and grill for about 30 minutes, turning the vegetables as needed so that they get browned in spots on all sides.
If you want to roast the vegetables in an oven, preheat the oven to 375°. Place a piece of foil on a baking sheet, place the tomatoes, peppers and garlic on the foil, and loosely close the foil over the top. Bake for about 1 hour until the vegetables are soft and a bit browned.
Transfer the vegetables and any juices that have accumulated in the foil to a food processor or blender. Add the cilantro, oregano (if using), lime juice, salt and pepper (if using). Pulse or puree, depending on how chunky you want the salsa to be. Taste and adjust the seasonings as needed.