Preheat your oven to 375°F (190°C). Line a baking sheet with parchment paper.
To create the cheddar cheese shells, spoon four heaps of shredded cheddar cheese (1/2 cup each) onto the baking sheet. Flatten them into circles, about 5 inches in diameter.
Bake for 6-8 minutes or until the edges turn golden brown. Watch closely to prevent burning.
Remove from the oven and let cool for 1-2 minutes. Using a spatula, lift the cheese circles and drape them over a spoon or rolling pin suspended between two cups to form a taco shell shape. Allow to cool completely to harden.
While the cheese is baking, heat olive oil in a skillet over medium heat. Add the chopped onion and cook until translucent, about 3-4 minutes.
Add the minced garlic and cook for an additional minute.
Increase the heat to medium-high and add the ground beef to the skillet. Cook until browned, breaking it apart with a spatula, about 5-7 minutes.
Drain excess fat and return the skillet to the stove. Stir in cumin, chili powder, paprika, cayenne pepper, salt, and pepper. Cook for another 2 minutes, stirring frequently.
Assemble the tacos by filling each cheese shell with the beef mixture. Top with diced tomatoes, sliced avocado, chopped cilantro, and a dollop of sour cream if desired.
Serve immediately and enjoy your keto-friendly Mexican feast!