Add tomato halves, onion quarters, serrano pepper, and garlic cloves to a glass bowl. Add olive oil and salt, then toss to coat.
3 Roma tomatoes, 1/2 red onion, 1 serrano pepper, 2 cloves garlic, 1 tbsp olive oil, 1 tsp kosher salt
Add oiled and salted tomatoes, garlic cloves, peppers and onion quarters to a baking sheet lined with foil.
Preheat oven to 400 degrees. Roast vegetables for 20 minutes.
Remove from the oven. Remove the tomato skins - they should come off easily.
Coarsely chop the onions, peppers, and tomatoes. Add the garlic, Tex-Mex spice blend, and lime juice to a molcajete. Crush with a tejolote until a paste forms. Add the onions and peppers, crush some more until well blended. Add the chopped tomatoes and cilantro, and crush until the desired consistency is achieved. It’s best to leave some chunks of tomato.