To make the pastry cups, press one wrapper into each of the wells of a mini muffin tin. If using spring roll wrappers, cut the wrapper into 3-4 inch squares and use one square per well. Bake for 3-5 minutes, or until the wrappers are golden brown and crispy. Carefully remove the mini cups from the muffin tin.
Combine the ground beef and onion in a skillet and cook over medium heat until browned, around 8 minutes. Remove from heat and cool.
Just before serving, fill each pastry cup with meat, tomato, cheese, avocado and cilantro. Top with a dollop of sour cream.
Notes
If using spring roll wrappers, these are usually around an 8×8″ square that can be cut into 4 pieces, so you will only need 3 spring roll wrappers to make 12 cups.
You can push a square of tin foil into the well to hold the wrapper open.