Step 1

For the taco shells: Place an oven rack in the upper third. Preheat the oven to 350 degrees F. Spray or brush a 12-cup muffin tin with olive oil.

Step 2

Wrap the tortillas in damp paper towels and microwave until hot and pliable, 15 to 20 seconds. Gently press one tortilla into each prepared muffin cup, pleating the sides and pressing gently to make a flat bottom. Once all of the tortillas are in the cups, spray lightly with olive oil and season with salt. Bake until the edges are browned and the cups are crisp throughout, about 12 minutes. Set aside.

Step 3

For the beef: Heat the olive oil in a large skillet over medium heat. Add the bell pepper, onion and a pinch of salt. Cook, stirring occasionally, until softened, about 4 minutes. Add the beef and season with 1/2 teaspoon salt. Cook, breaking up the meat into smaller pieces with a wooden spoon, until browned, 4 to 5 minutes. Spoon or pour off all but 2 tablespoons of fat. Make a space in the center of the pan and add the tomato paste. Toast the tomato paste for a minute, then sprinkle with the chili powder, cumin, paprika, garlic powder and oregano. Stir to incorporate. Add the beef broth, bring to a boil and reduce to a simmer. Cook until thickened, about 5 minutes.

Step 4

To serve, divide the beef among the taco cups and top with avocado, sour cream, tomato, Cheddar, scallions and shredded lettuce. Serve with lime wedges on the side.