In a food processor, combine 1.5 cups of all-purpose flour and 0.25 teaspoon of salt. Pulse in 10 tablespoons of very cold unsalted butter until the mixture resembles coarse meal. Add 1 large egg and 2 tablespoons of cold ice water; pulse until clumps form. Wrap the dough in plastic wrap and refrigerate for at least 1 hour.

Roll out the dough on a floured surface to fit a 9-inch pie dish. Transfer the dough to the dish, press it into the corners, and trim any excess. Flute the edges for decoration and return to the refrigerator.

Heat 1 tablespoon of olive oil in a skillet over medium heat. Add 1 pound of chopped shrimp and sauté until cooked, about 2-3 minutes. Then, add 1 cup of finely chopped spinach, 0.5 cup of diced tomato, 2 tablespoons of chopped green onions, and 2 minced garlic cloves. Cook for an additional 2 minutes.

Preheat the oven to 350°F (175°C). In the prepared crust, layer 2 cups of shredded Gruyère cheese at the bottom. Spoon the sautéed shrimp and vegetable mixture over the cheese. In a bowl, whisk together 3 large eggs, 1.5 cups of whole milk, 0.5 teaspoon of sea salt, and 0.125 teaspoon of black pepper. Pour the custard mixture over the filling.

Place the quiche in the preheated oven on a baking sheet. Bake for 50-60 minutes, until the center is slightly jiggly and the edges are golden brown. Allow to cool for about 15 minutes before slicing and serving.