Place the all-purpose flour and pinch of salt into the bowl of a food processor. Pulse twice to combine.

Drop the deeply chilled, cubed butter evenly over the flour. Pulse the machine repeatedly (about 10 seconds) until the fat is cut into the flour and the mixture resembles coarse, pea-sized crumbs. (Preserving these cold lipid pockets is critical for a flaky crust).

With the machine running continuously, slowly stream the ice water through the feed tube just until the dough begins to clump together and pull away from the sides. Stop immediately.

Turn the shaggy dough out, gather it into a tight ball, flatten it into a 1-inch thick disk, and wrap tightly in plastic wrap. Refrigerate for a minimum of 1 hour to completely relax the gluten and solidify the butter.

Preheat the oven to 400°F. Roll the chilled dough out on a floured surface to a ¼-inch thickness. Press it firmly into a 9-inch fluted tart pan with a removable bottom.

Pierce the bottom of the raw crust repeatedly with a fork. Bake the empty crust (blind bake) until lightly golden and structurally set, about 9 to 12 minutes. Remove and set aside.

In a large skillet over medium-high heat, cook the 1-inch bacon pieces until deeply crisp and the fat has rendered. Use a slotted spoon to transfer the bacon to paper towels. Carefully pour off the excess fat.

In the same skillet, melt the olive oil and 1 tablespoon of butter over medium-high heat. Add the diced yellow onion and cook until soft, about 5 minutes.

Drizzle the balsamic vinegar over the onions and cook for an additional 4 to 5 minutes until the onions are deeply golden and slightly syrupy. Transfer to a bowl.

Add the fresh spinach to the hot skillet and toss gently until just wilted, about 5 minutes. Season the spinach heavily with the Herbs de Provence.

Carefully slide the pre-baked fluted tart shell onto a sturdy baking sheet for easy transfer.

Scatter the crisp bacon pieces evenly across the bottom of the baked tart shell. Cover the bacon with an even layer of the grated smoked Fontina cheese. (Fontina is an exceptional melting cheese that binds the base securely).

Cover the cheese with the balsamic-caramelized onions, followed by the wilted spinach.

In a medium bowl, vigorously whisk together the extra-large eggs, heavy whipping cream, kosher salt, coarse black pepper, and ground nutmeg.

Carefully pour this liquid custard entirely over the layered ingredients in the tart shell, allowing it to seep deep into the crevices.

Bake in the 400°F oven until the egg mixture puffs beautifully and is deeply golden brown and fully set in the center, about 30 to 40 minutes.

Allow the tart to cool to room temperature to stabilize the custard before releasing the fluted ring and slicing.