Heat a large cast-iron skillet over medium-high heat for about 2 minutes.
Add tomatoes, onion, garlic, and halved chiles and toast the vegetables, using tongs to turn them frequently, until softened and almost completely blackened, about 10 minutes for the tomatoes and 15 to 20 minutes for the other vegetables.
When cool enough to handle, trim the root ends from the onion and halve each piece.
Remove the skins from the garlic and mince.
Mice the jalapenos.
Adjust an oven rack to the middle position and heat oven to 350°F.
Process the toasted tomato and onion in a food processor until smooth and thoroughly pureed, about 15 seconds, scraping down bowl if necessary.
Transfer mixture to a liquid measuring cup; you should have 2 cups. If necessary, spoon off any excess so that the volume equals 2 cups.
Follow the recipe for Mexican Rice from heating the oil in the pan or Dutch oven, adding the toasted minced jalapenos and garlic along with the pureed tomato mixture.