Combine sugar, flour, oats, and salt in a bowl. Using a fork or a pastry blender, cut butter into the mixture until it is crumbly and resembles coarse meal. Set aside.

Combine blueberries, sugar, flour and lemon juice in a bowl and mix well with a spatula. Set aside.

In a food processor, add butter, sugar, flour, and salt. Pulse until the mixture comes together and forms small clumps.

Transfer out the dough and divide it into 12 equal pieces. Place each piece into each muffin hole. Feel free to grease the muffin tin with non-stick cooking spray or melted butter. Mine was not greased and they came out just fine.

Using fingers, press dough against the bottom and the sides of muffin hole to form a mini tart shell.

Add the blueberry filling into each tart, then top with the oat crumbles.

Bake at 375°F / 190°C for 20 - 22 minutes or until tart shell is golden brown.

Using a toothpick, poke the side of tart to scoop it out of the pan.