Preheat your oven to 375°F (190°C). Roll out the pastry and line a 9-inch tart pan with removable bottom. Prick the bottom with a fork, line with parchment paper, and fill with pie weights. Blind bake for 15 minutes, remove weights, and bake another 5 minutes until lightly golden. Set aside to cool.

Heat butter and olive oil in a large skillet over medium heat. Add sliced onions and a pinch of salt. Cook, stirring occasionally, for 30-40 minutes until deeply caramelized and golden brown. Be patient and keep the heat at medium-low to avoid burning.

When onions are caramelized, add minced garlic and fresh thyme leaves. Cook for another 2 minutes until fragrant. Stir in balsamic vinegar and sugar, cook for 1 minute more. Season with salt and pepper to taste. Remove from heat and let cool slightly.

In a mixing bowl, whisk together eggs, heavy cream, milk, nutmeg, salt, and pepper until well combined and smooth.

Spread the caramelized onion mixture evenly over the blind-baked tart crust. Sprinkle the grated Gruyere cheese over the onions, distributing it evenly.

Carefully pour the egg and cream mixture over the onions and cheese, filling to just below the rim of the crust. Tap the pan gently on the counter to release any air bubbles.

Bake the tart for 30-35 minutes until the filling is set but still slightly jiggly in the center. The top should be golden brown and the cheese fully melted.

Remove from oven and let cool on a wire rack for at least 15 minutes before removing from the tart pan. Garnish with fresh microgreens or additional thyme sprigs.

Slice into wedges and serve warm or at room temperature. The tart can be made ahead and reheated gently in a 300°F oven for 10 minutes.