Special equipment: a 12-cup mini muffin tin
Preheat the oven to 400 degrees F. Put a 12-cup mini muffin tin near the oven. Line a baking sheet with a nonstick baking mat or parchment.
Combine the Cheddar and taco seasoning in a medium bowl and mix well.
Sprinkle 1 tablespoon of the cheese mixture into a mound on the prepared baking sheet. Make 11 more cheese mounds on the baking sheet, leaving about 3 inches of space between each mound. Press each down slightly so that they are about 1/2 inch thick.
Bake until the cheese mounds fall into flat disks and the edges are golden brown, about 4 minutes. Working quickly, before the cheese crisps and hardens, transfer the cheese disks with a small offset spatula to the cups of the mini muffin pan and press each into the bottom and against the sides to form cups. Let cool completely in the tin and then transfer to a serving platter.
Once the baking sheet has cooled completely, make another batch of cheese crisps with the remaining cheese mixture.
Combine the onion, tomato, cilantro, oil, lime juice, avocados and salt in a bowl and mix gently to combine.
Fill each cheese cup with some of the guacamole.