Peel and cube the sweet potatoes into about ½-inch cubes.

Boil them in a pot of 6 cups of lightly salted water for about 20-30 minutes.

Dice the onions and add them to a medium-hot pan with the butter.

Caramelize the onions for 20-30 minutes, or until they are soft and browned.

Keep stirring them in the pan to prevent charring.

After the onions are caramelized and the sweet potatoes are soft enough to be broken with a fork, add them to a large glass mixing bowl.

Add the Parmesan, salt, and cracked black pepper.

Blend the ingredients with an immersion blender, food processor, or normal blender.

Blend until everything is combined and smooth.

Place the filling in a piping bag and pipe it into the tartlet shells until they are filled.