Heat a cast iron skillet large enough to hold the vegetables in a single layer over medium heat. Let it get nice and hot. Add the tomatoes, onion wedges, and serrano chile. Let cook undisturbed for 5 minutes, until beginning to char. With tongs, flip and continue trying on all sides until everything is nicely blistered, about 10 minutes more.

Transfer the vegetables to a blender. Add the garlic, lime juice, salt, cumin, and most of the cilantro. Pulse in short burst until the salsa reaches your preferred consistency. It does not take much blending. Taste and add salt as desired. Pour into a bowl, sprinkle with remaining cilantro, and refrigerate until ready to serve. Enjoy with chips, tacos, and any of your favorite Tex-Mex dishes.