Preheat the oven to 375°F (190°C) and line a baking sheet with parchment paper.
Place small heaps of cheddar cheese (about 1/4 cup each) on the parchment paper, spaced well apart. Flatten them slightly into circles.
Bake for 5-7 minutes, or until the edges are golden brown and crispy.
Remove from the oven and allow them to cool for 2-3 minutes before carefully lifting with a spatula and draping over a spoon or rolling pin to form a taco shell shape. Allow them to cool and harden.
Heat olive oil in a skillet over medium heat. Add diced onion and cook until translucent, about 3-5 minutes.
Add minced garlic and cook for an additional minute until fragrant.
Add the ground beef to the skillet, breaking it apart with a spoon. Cook until browned, about 5-7 minutes.
Drain excess fat if necessary, then stir in taco seasoning and water. Simmer for 5 minutes until the mixture thickens. Season with salt and pepper to taste.
Assemble the tacos by filling each cheese shell with the seasoned beef.
Top with sliced avocado, a dollop of sour cream, chopped cilantro, and sliced jalapeño if desired.
Serve immediately for best texture.