In a large pot, heat the oil over high heat. Once the oil is glistening, add the tomatoes, jalapeños, 1 half of the white onion, and garlic in a single layer. Cook undisturbed, until the vegetables are charred, about 3-5 minutes. Turn and repeat on the other side until charred, about 3-5 minutes more.
Reduce the heat to low and cover with a lid. Simmer, covered, for 15 minutes, or until mostly tender. Remove from heat and allow to cool, about 15 minutes.
To a food processor, add the charred vegetables (starting with only 1 jalapeño), crushed tomatoes, cilantro, salt, and the remaining half of the white onion. Blend on low until the desired texture is achieved. Add remaining jalapeños, one at a time, to increase the spice level. Add the avocado and pulse until just combined.
Transfer to a bowl, adjust salt as needed. Store for up to 1 week refrigerated in an airtight container.
