Preheat the oven to 400°F / 205°C. Cut 12 corn tortillas into eighths. Spread on lightly oiled baking sheets, brush with oil and bake for 10-15 minutes, until crisp.
Stem and seed 9 guajillo chilies. Cover with water in a pot and bring to a boil. Remove from the heat, cover and soak for 10 minutes.
Lift the chilies into a blender. Add 1½ cups of fresh water, 1 Roma tomato, 1 garlic clove, ¼ medium white onion, and 1½ teaspoons of kosher salt. Blend until smooth; strain if any skin remains.
Heat 1½ teaspoons of vegetable oil in a large skillet. Add the sauce and simmer for 3 minutes, stirring.
Remove from the heat and gently fold in the chips. Serve immediately, with cotija cheese and Mexican crema if desired.
