Prepare Crust & Oven: If using a homemade crust, ensure it is well-chilled. Position an oven rack in the center and preheat your oven to 400∘F (205∘C).

Blind Bake Crust: On a lightly floured surface, roll out the pie dough to approximately a 12-inch diameter circle. Gently ease the dough into a 10-inch pie dish. Trim any excess dough and create a decorative edge (e.g., crimp or flute). Line the dough snugly with parchment paper (tip: crumple the paper first for a better fit) or aluminum foil. Fill evenly with pie weights or dried beans. Bake for 15-17 minutes, observing until the crust edges begin to color lightly.

Finish Blind Baking: Carefully remove the parchment/foil containing the weights/beans. Using the tines of a fork, prick the bottom surface of the crust multiple times. Return the empty crust to the oven for an additional 7-9 minutes, baking until the bottom looks dry and has taken on a pale golden hue. Place the pre-baked crust on a wire rack to cool slightly. Reduce the oven temperature to 350∘F (175∘C).

Caramelize Onion: While the crust is baking, warm the olive oil in a large, heavy-bottomed skillet over medium-low heat. Add the chopped yellow onion. Cook slowly, stirring infrequently at first, then more often as it colors, until the onion is exceptionally tender, deeply golden brown, and sweet – properly caramelized. This crucial step takes patience, typically 30-40 minutes.

Cook Aromatics & Spinach: Introduce the chopped spinach to the skillet with the caramelized onions, adding it in batches if necessary, stirring until just wilted. Mix in the minced garlic, kosher salt, and ground black pepper. Continue to cook and stir for about 1 minute more, just until the garlic becomes fragrant. Take the skillet off the heat.

Mix the Filling: In a separate medium-large bowl, whisk the large eggs and milk together thoroughly. Add the warm onion-spinach mixture from the skillet, the crumbled goat cheese, the diced roasted red peppers, and the chopped sun-dried tomatoes to the egg mixture. Gently fold everything together until combined; avoid overmixing.

Assemble and Bake Tart: Pour the prepared filling carefully into the pre-baked tart shell, arranging the ingredients evenly. For easier handling and to catch any drips, place the pie dish onto a larger baking sheet. Bake in the preheated 350∘F (175∘C) oven for 55 to 70 minutes. The tart is done when the filling is puffed and golden, and the center appears set (it should not wobble excessively when the dish is gently nudged). A thin knife inserted about an inch from the center should come out clean.

Cool Before Serving: Remove the tart from the oven and let it cool on a wire rack for a minimum of 15 minutes before slicing. This allows the filling to fully set. Serve the tart warm or allow it to cool to room temperature.

Estimated Nutritional Information (per serving, based on 8 servings):