Heat half the olive oil over medium-high heat in a skillet. Add the peppers and onions. Cook for 7-9 minutes until tender-crisp. If the peppers and onions are cooking too quickly, add a couple of tablespoons of water. Remove and set aside.
Add the remaining oil. Add the beans, salsa, water, salt, and pepper. Cook for 5-7 minutes, stirring as needed. Use a potato masher or back of a spoon to mash. If the beans are thicker than you would like, add some extra water or broth. Taste and season so they are very flavorful.
To make the tostadas: Preheat the oven to 400 degrees. Place the tortillas on a baking sheet or directly on the oven rack. Cook for about 4 minutes on each side until they are crispy. You can cook these at the same time as the beans.
Assemble the tostadas: Spread the beans on the tostada shells. Add the sauteed peppers and onions. Then add all your favorite toppings. I like mine with sliced avocado, cilantro, tomatoes, jalapeno, and queso fresco.