Whether cooking the beans from scratch or using canned, drain the beans and add them to a saucepan over low heat and warm through. Add about 1/4 cup tepid water, 1/4 cup of the sour cream, the green onions, cumin, salt, and pepper and mash with a potato masher or a large fork until coarsely mashed but not entirely smooth. Stir in the bacon. Taste for salt and pepper, add the remaining sour cream if you'd like the beans to be creamier, then turn off the heat and keep covered until needed.
Preheat the oven to 400 degrees F. Brush the tops of the tortillas with a bit of olive oil and lay them on a rimmed baking sheet (it's fine if they overlap). Bake until just lightly browned, 6 to 8 minutes. Remove from the oven and set aside.
Heat a large frying pan with the coconut oil over medium heat. Working in batches as necessary, gently break the eggs in the pan and cook sunny-side up or to your desired doneness, covering the pan if you like your yolks more cooked through.
Build a tostada by topping a tortilla with about 1/4 cup of the bean mash, 1 egg, and an eighth of the cheese, avocado slices, and cilantro. Repeat with the remaining ingredients. Serve garnished with a slice of lime and hot sauce to taste.