Preheat the oven to 450°F. Wash and dry the fresh produce. Thinly slice the scallions, separating the white bottoms and hollow green tops. Drain and rinse the beans. Halve the tomatoes. Halve, peel, and medium dice the onion(s). Cut off and discard the stem of the pepper(s). Halve lengthwise; remove the ribs and seeds, then medium dice. Thoroughly wash your hands immediately after handling. In a bowl, combine the guacamole and 1 tablespoon of the sour cream (or 2 tablespoons for 4 servings); season with salt and pepper.

In a medium nonstick pan (or large for 4 servings), heat a drizzle of olive oil on medium-high until hot. Add the sliced white bottoms of the scallions; season with salt and pepper. Cook, stirring constantly, 30 seconds to 1 minute, or until slightly softened. Carefully add the drained beans and 1/4 cup of water (or 1/2 cup for 4 servings); season with salt and pepper. Cook, stirring frequently and mashing the beans with a potato masher (or the back of a spoon), 4 to 5 minutes, or until slightly thickened. Transfer to a bowl; stir in the remaining sour cream. Taste, then season with salt and pepper if desired. Cover with foil to keep warm. Rinse and wipe out the pan.

Place the tortillas on a sheet pan. Using a fork, prick the tortillas all over to prevent air bubbles from forming. Drizzle with olive oil and season with salt and pepper; turn to coat. Arrange in an even layer (it's okay if they're overlapping). Toast in the oven 4 to 6 minutes, or until lightly browned and crispy. Remove from the oven.

Meanwhile, in the same pan, heat a drizzle of olive oil on medium-high until hot. Add the sausage; break the meat apart with a spoon. Cook, without stirring, 3 to 4 minutes, or until lightly browned. Continue to cook, stirring frequently, 4 to 5 minutes, or until browned and cooked through. Leaving any browned bits (or fond) in the pan, transfer to a plate. Cover with foil to keep warm. Use the same pan to cook the vegetables.

In the same pan, heat a drizzle of olive oil on medium-high until hot. Add the halved tomatoes, diced onion(s), and diced pepper(s) in an even layer. Cook, without stirring, 1 to 2 minutes, or until lightly browned. Carefully add the sugar, 2 tablespoons of water (or 4 tablespoons for 4 servings), and as much of the chile paste as you'd like, depending on how spicy you'd like the dish to be. Season with salt and pepper. Cook, stirring occasionally, 3 to 4 minutes, or until the vegetables are coated and softened. Turn off the heat. Taste, then season with salt and pepper if desired.

Assemble the tostadas using the toasted tortillas, mashed beans, cooked sausage, and cooked vegetables. Serve the tostadas topped with the creamy guacamole. Garnish with the cotija and sliced green tops of the scallions. Enjoy!