Add the All-Purpose Flour (1 1/4 cup), Salt (1/2 teaspoon), and Granulated Sugar (1 teaspoon) to a food processor. Add the Unsalted Butter (8 tablespoon) and pulse until the butter is distributed into pea-sized pieces.
Add the Non-Fat Milk (5 tablespoon) one tablespoon at a time, pulsing as you go, until the mix forms a rustic ball.
On a lightly floured surface, shape dough into a 5-inch disk. Wrap in plastic and refrigerate for 1 hour.
Preheat oven to 400 degrees F (200 degrees C).
On a floured board, allow dough to sit, unwrapped, until just pliable, about 15 minutes.
With a floured rolling pin, roll dough into a large circle, moving it a quarter turn with every roll to make sure it's not sticking underneath. In the end, dough should be 1/4-inch thick.
Ease dough round into a 9-inch tart pan with a removable bottom. Press tightly into the sides of the pan; the dough should exceed the edge by 1/2 inch. Using the rolling pin, cut excess dough against the tart pan to create a perfectly even edge. Chill 30 minutes before cooking.
With the tines of a fork, poke holes throughout the bottom of the crust.
Cover with foil and top with pie weights or uncooked beans or rice. Bake 15-20 minutes, or until edge of crust is light golden.
Remove crust from oven and carefully remove weights and foil. Return to oven to bake 5-10 minutes more. Cool on rack. Keep oven on at 400 degrees F (200 degrees C).
Add Olive Oil (2 tablespoon) to a saute pan and heat to medium-low heat.
Add the Onion (1) cook, tossing around every few minutes. Once the onions become translucent, reduce the heat to low. Cook 20-25 minutes until caramelized. Season the onions with Kosher Salt (1 teaspoon) and Freshly Ground Black Pepper (1/2 teaspoon). Set aside until needed.
Add the remaining Olive Oil (1 tablespoon) to a separate small saute pan. Cook the Garlic (2 clove) and Fresh Thyme Leaves (2 teaspoon) until fragrant, about 30 seconds.
Add the Heavy Cream (1/2 cup). Increase the heat until mixture is reduced by half. Stir in Gruyère Cheese (3/4 cup).
Pour reduced liquid into prepared pie shell.
Top with onions and Beefsteak Tomato (0.5 pound), then sprinkle with remaining Gruyère Cheese (1/4 cup). Cook until tart is hot and cheese is melted, 10-15 minutes.
Place under the broiler for 5 minutes before serving.
Garnish with Fresh Basil (to taste).