Halve and peel onion. Slice halves into thin strips. Place a medium non-stick pan over medium-high heat. Add 2 tsp. olive oil, onions, and a pinch of salt and pepper to hot pan and stir occasionally until softened, 5 minutes. Add ¾ cup water and cranberry sauce and stir occasionally until mix is a jam-like consistency, 8-12 minutes. Transfer onions to a plate. Wipe pan clean and reserve. While onions caramelize, prepare ingredients.
Prepare the Ingredients
Trim stems off Brussels sprouts and thinly slice. Halve grape tomatoes. Coarsely chop smoked almonds.
Prepare the Filling
In a mixing bowl, thoroughly combine Brussels sprouts, tomato, half the almonds (reserve remaining for garnish), 2 tsp. olive oil, ¼ tsp. salt, and a pinch of pepper.
Make the Tart Crust
Remove puff pastry from refrigerator and remove paper from between sheets. Place pastry on prepared baking sheet. Fold each side ½" over on top of square. Using two fingers, pinch sides to make a decorative edge.
Assemble and Bake Tarts
Divide mozzarella cheese equally between four puff pastry squares, then divide caramelized onions equally. Place a mound of Brussels-tomato mixture on each tart. Don't worry if mixture spills over the edge. Bake until pastry is golden brown and filling is beginning to char, 20-25 minutes, rotating baking sheet after 10 minutes. Garnish with remaining almonds and blue cheese and enjoy!