Heat 1 tablespoon olive oil in a frying pan over medium heat.
Process the chopped onion in a food processor or blender until it forms a paste. Add to the hot frying pan.
Sauté the onion paste for about 1 minute, then add the ground sirloin, cumin, and cayenne pepper.
Cook the beef mixture until browned, then season with salt and pepper to taste. Set aside.
While the beef cooks, prepare all toppings: grate the cheeses using a fine grater, cube the avocado, chop the tomatoes, and shred the lettuce.
Heat 2 cups of cooking oil in a frying pan over medium-high heat, ensuring there's enough oil to completely submerge a taco shell.
Test oil temperature by dropping a small piece of taco shell into the oil - it should sizzle immediately when ready.
Using tongs, carefully place one taco shell in the hot oil and immediately shape it into a taco form. Cook until crispy and golden.
Remove the fried shell with tongs and place upside down on paper towels to drain excess oil.
Repeat with remaining shells, working one at a time.
Fill each crispy shell with the seasoned beef and serve with prepared toppings for a taco bar experience.