Heat the oil in a skillet over medium heat. Add the onion and cook for 3–4 minutes until softened. Stir in the garlic and cook 30 seconds more.
Add the beans, cumin, chili powder, and the reserved bean liquid. Mash with a potato masher or fork until mostly smooth but still a little textured. Cook for 3–4 minutes, stirring, until thick and creamy. Season with salt and pepper.
Warm the tortillas briefly so they're pliable. Spread about 1/3 cup of the bean mixture down the center of each tortilla, then top with a generous handful of shredded cheese.
Fold in the two short sides, then roll tightly from the bottom up, tucking the filling in as you go, until you have a sealed burrito with the seam on the bottom
Melt a little butter in a large skillet over medium heat. Place the burritos seam-side down and cook for 2–3 minutes per side, until golden and crisp, and the cheese inside is fully melted.
Let rest for a minute, then slice in half and serve with salsa, sour cream, or a squeeze of lime.
