Pour vegetable oil into a heavy skillet or small pot to a depth of about 1/8–1/4 inch (enough to shallow-fry). Heat over medium-high until shimmering and a small pinch of masa sizzles (about 350°F if using a thermometer). Fry each blue corn tortilla 30–45 seconds per side until crisp and slightly blistered. Transfer to a paper towel-lined rack and immediately season lightly with salt. Keep warm in a low oven (200°F / 95°C) on a baking sheet while finishing other components.

Refried beans

Cook aromatics

Heat 1 tablespoon vegetable oil in a medium skillet over medium heat. Add the finely chopped onion and cook, stirring occasionally, until translucent, about 4–5 minutes. Add the minced garlic and cook 30–45 seconds until fragrant.

Mash and season beans

Add the drained pinto beans to the skillet with the onion and garlic. Stir to combine, then add the ground cumin and salt. Use a potato masher or fork to mash the beans to your desired texture (smooth with some chunkiness is traditional). If the mixture seems dry, add up to 1/4 cup water or reserved bean liquid a little at a time to reach a spreadable consistency.

Finish beans

Cook the mashed beans, stirring frequently, 3–5 minutes so flavors meld and excess moisture evaporates. Taste and adjust salt or cumin as needed. Keep warm until assembly.

Toppings

Prepare toppings

Shred the cheddar cheese and shred or thinly slice the lettuce. If using cilantro, chop finely. Have the mild salsa at room temperature or slightly chilled and cut lime wedges.

Assemble tostadas

Place a crisp blue corn tortilla on a plate. Spoon about 1/4 to 1/3 cup of the warm refried beans onto each tortilla and spread evenly, leaving a small border. Sprinkle about 1/4 cup shredded cheddar over the beans so it begins to melt from the warmth. Top with a generous handful (about 1/4 cup) of shredded lettuce. Spoon 1–2 tablespoons of mild salsa over the lettuce. Garnish with a sprinkle of chopped cilantro and serve with a lime wedge on the side.

Serve

Serve immediately while the tostada is crisp. If preparing for a group, keep fried tortillas in a warm oven and top individual tostadas just before serving to maintain crunch.