Cut the bell peppers through the stem into sixths, remove the stem and seeds. Transfer to a large microwave safe bowl, add a splash of water and a pinch of salt. Cover and microwave until the pepper pieces are pliable, about 4 minutes. Let cool slightly and then arrange close together on a foil-lined baking sheet, cut sides-up.
Meanwhile, heat the oil in a large nonstick skillet over medium-high heat. Add the chili powder and cumin and cook, stirring, until fragrant and toasted, about 30 seconds. Add the ground beef and 1/4 teaspoon salt and cook, stirring and breaking up into bite-size pieces, until browned and cooked through, about 4 minutes.
Preheat the broiler. Spoon some beef mixture onto each pepper piece. Sprinkle with cheese and broil until the cheese melts, about 1 minute. Top with dollops of guacamole and pico de gallo. Thin the sour cream out with a little water and drizzle over the nachos.
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