Step 1: Prepare the Pastry

1. Preheat and Prep: Preheat your oven to 400°F (200°C). Lightly grease a standard 12-cup muffin tin.

2. Roll and Cut: Gently unfold your thawed puff pastry sheet onto a lightly floured surface. If needed, use a rolling pin to lightly roll it out to ensure it’s an even thickness (about 1/8-inch). Using a 4-inch round cookie cutter (or the rim of a wide glass), cut out 12 circles from the pastry sheet.

3. Form the Shells: Carefully press each pastry circle into a cup of the prepared muffin tin. Gently guide the pastry down to line the bottom and sides of each cup. Don’t worry if it doesn’t reach the very top edge perfectly.

4. Dock the Pastry: Using a fork, prick the bottom of each pastry shell a few times. This is called “docking” and it prevents the base of the pastry from puffing up too much during baking, leaving ample room for the filling.

5. Par-Bake (Optional but Recommended): For an extra-crispy crust, you can “par-bake” or “blind-bake” the shells. Place the muffin tin in the oven and bake for 8-10 minutes, or until the pastry is lightly puffed and just beginning to turn golden. Remove from the oven and set aside. This step helps ensure a crisp, non-soggy bottom.

Step 2: Cook the Bacon and Spinach

1. Crisp the Bacon: Place the chopped bacon in a medium-sized skillet over medium heat. Cook, stirring occasionally, until the bacon is brown and crispy, about 7-9 minutes.

2. Drain the Bacon: Using a slotted spoon, remove the crispy bacon from the skillet and transfer it to a plate lined with paper towels to drain any excess grease. Reserve about 1 tablespoon of the rendered bacon fat in the skillet and discard the rest.

3. Sauté the Aromatics: Add the finely minced shallot to the skillet with the reserved bacon fat. Sauté for 2-3 minutes until softened and translucent. Add the minced garlic and cook for another minute until fragrant.

4. Wilt the Spinach: If using fresh spinach, add it to the skillet in batches. Cook, stirring, until it has completely wilted down. If using thawed frozen spinach, add it to the skillet and heat through.

5. Squeeze Dry: Transfer the spinach mixture to a fine-mesh sieve or a bowl. Once cool enough to handle, use your hands or the back of a spoon to press out ALL excess liquid. This step is absolutely critical for the final texture of your tartlets. Roughly chop the squeezed spinach.

Step 3: Create the Creamy Egg Custard

1. Whisk the Base: In a medium bowl, whisk together the 3 large eggs until the yolks and whites are fully combined and slightly frothy.

2. Add Cream and Seasoning: Pour in the heavy cream and whisk to combine. Add the salt, black pepper, and the pinch of nutmeg. Whisk until everything is smooth and homogenous.

Step 4: Assemble Your Tartlets

1. Layer the Flavors: Now for the fun part! Sprinkle a small amount of the grated Gruyère cheese into the bottom of each pastry shell.

2. Add the Solids: Evenly distribute the crispy bacon pieces and the squeezed, chopped spinach mixture among the 12 tartlet shells.

3. Top with More Cheese: Sprinkle the remaining Gruyère and all of the grated Parmesan cheese over the top of the fillings.

4. Pour the Custard: Carefully pour the prepared egg custard into each tartlet shell, filling them about three-quarters of the way full. Be careful not to overfill, as the custard will puff up as it bakes.

Step 5: Bake to Golden Perfection

1. Bake: Place the muffin tin on a baking sheet to catch any potential spills. Bake in the preheated 400°F (200°C) oven for 20-25 minutes.

2. Check for Doneness: The tartlets are done when the pastry is deep golden brown and flaky, and the egg filling is puffed, set, and lightly golden on top. A knife inserted into the center of a tartlet should come out clean.

3. Rest and Serve: Remove the muffin tin from the oven and let the tartlets cool in the tin for 5-10 minutes. This allows the filling to set further and makes them easier to remove. Run a thin knife around the edge of each tartlet to loosen, then carefully lift them out. Serve warm.