1. Preheat oven to 425ºF. Rub potatoes with 1 Tbsp. oil; sprinkle with salt. Microwave uncovered on high for 6 minutes. Turn; microwave until just cooked through, 3 to 5 minutes longer.
2. Transfer potatoes to a baking sheet. Bake until skins are crisp, 20 minutes. Warm remaining oil in a skillet over medium-high heat. Sauté onion until softened, about 3 minutes. Add garlic; sauté 30 seconds. Add chili powder; cook, stirring, 1 minute. Add beans and tomatoes; cook until heated through, stirring occasionally, about 3 minutes. Season with salt and pepper. Keep warm.
3. Cut a slit in top of each potato; squeeze sides to open. Spoon bean mixture on top of potatoes, sprinkle with cheese and scallions and serve.