In a medium bowl combine 1.5 cups all-purpose flour, 1 tsp salt and 0.5 tsp sugar. Cut 6 tbsp cold unsalted butter into small cubes and work into the flour with a pastry cutter or fingers until mixture resembles coarse crumbs with pea-size pieces of butter remaining.

Add water and chill

Sprinkle 3 tbsp cold water over the flour mixture, tossing gently with a fork until the dough just comes together. Shape into a disk, wrap in plastic, and refrigerate for at least 30 minutes (up to 2 days) before rolling.

Blind bake crust

Preheat oven to 375°F (190°C). On a lightly floured surface roll chilled dough into a 10-inch circle about 1/8" thick. Fit dough into a 9-inch tart pan with removable bottom, trimming excess. Prick the bottom lightly with a fork. Line crust with parchment and fill with pie weights or dried beans. Bake 15 minutes, remove weights and parchment, then bake 6–8 minutes more until edges are lightly golden. Let cool slightly.

Filling

Prepare sundried tomatoes and spinach

If using oil-packed sundried tomatoes, drain and chop into 1/4–1/2" pieces (use 3/4 cup). If dry-packed, rehydrate in hot water for 10 minutes, drain and toss with 1 tbsp olive oil. Roughly chop 4 cups fresh spinach.

Sauté aromatics & spinach

Heat a tablespoon-sized skillet or sauté pan over medium heat with 1 tsp olive oil (or a splash from the sundried tomato oil). Add 1 small finely chopped yellow onion and cook 4–5 minutes until soft and translucent. Add 2 minced garlic cloves and cook 30 seconds until fragrant. Add the chopped spinach and sauté 2–3 minutes until wilted and any excess liquid is evaporated. Remove from heat and let cool 5 minutes.

Whisk custard

In a bowl whisk together 3 large eggs and 3/4 cup heavy cream until smooth. Stir in 1/2 cup grated Parmesan, 3/4 cup shredded Gruyère or mozzarella, 2 tbsp thinly sliced fresh basil, 1/2 tsp dried oregano, 1 tsp salt, 1/2 tsp black pepper and 1/4 tsp red pepper flakes if using.

Assemble tart

Evenly scatter the chopped sundried tomatoes over the partially baked crust. Spread the sautéed spinach and onion mixture over the tomatoes. Pour the egg and cheese custard over the vegetables, pressing gently so custard fills gaps. Smooth surface and sprinkle additional 2 tbsp grated Parmesan over top.

Bake filling

Reduce oven to 350°F (175°C). Bake the assembled tart on the middle rack for 25–30 minutes, until center is set (slight jiggle is okay) and top is golden. If edges brown too quickly, tent loosely with foil for the last 8–10 minutes.

Rest

Remove tart from oven and let rest on a wire rack 10–15 minutes to finish setting before slicing. This improves slice quality.

Finish / Garnish

Brush the warm tart crust lightly with 1 tsp olive oil for shine. Scatter 4 fresh basil leaves (torn) over the top and finish with additional grated Parmesan to taste. Slice into 6–8 pieces and serve warm or at room temperature.