Combine flour and salt in a bowl or food processor. Pulse or cut in the cold butter until coarse meal forms with some pea-sized pieces. Add ice water 1 tablespoon at a time, mixing until dough forms. Form a disc, wrap in plastic, and refrigerate for 1 hour.

Sauté sliced red onion in butter and olive oil over medium-low heat with a pinch of salt. Cook, stirring occasionally, for 25-30 minutes until deeply caramelized and amber-brown. Deglaze with balsamic vinegar and add pepper. Let cool.

Wash and dry asparagus. Trim woody ends. Select 8-10 best spears, toss with a drizzle of olive oil, balsamic vinegar, and flaky salt. Reserve a few caramelized onions for dynamic visual garnish. Finely chop interior asparagus.

Preheat oven to 400°F (200°C). Roll out chilled dough to a 12-inch circle. Transfer to a 9-inch tart pan with removable bottom. Press in, trim excess, and prick all over with a fork. Line with parchment and fill with pie weights or beans. Bake for 15 minutes. Remove weights and parchment and bake 5-7 minutes more until deeply golden. Let cool slightly.

Lower oven temperature to 375°F (190°C). Gently whisk together eggs, extra yolk, heavy cream (or half-and-half), fine sea salt, garlic powder, nutmeg, black pepper, and flaky sea salt. Stir in the freshly grated sharp Gruyère cheese.

Spread cooled complex caramelized red onions and interior asparagus evenly over the bottom of pre-baked, cooled flaky crust. Gently pour the silky savory egg and Gruyère custard over the vegetables.

Carefully arrange your vibrant green asparagus spears and sophisticated red onion halves in a precise starburst pattern that radiates out from the center point. Tossing the top vegetables in the complex balsamic glaze provides a dynamic visual finish. Top with unexpected sprinkle of red pepper flakes and a final crack of savory black pepper. Place tart pan on a baking sheet and bake for 35-45 minutes until custard is set but slightly jiggling in center and the top is flawlessly golden-brown. Let cool 10-15 minutes before serving garnished with a handful of fresh microgreens.