Caramelize the Onions:

1. Slice Onions: Peel and thinly slice onions into ¼-inch half-moons. Uniform slicing ensures even cooking.
2. Start Cooking: Heat butter and olive oil in a large skillet over medium heat. Add sliced onions and salt. Stir to coat onions in fat.
3. Caramelize: Cook onions for 40-50 minutes, stirring every 5-10 minutes, reducing heat to medium-low after 15 minutes. Onions should become golden brown, jammy, and sweet. If onions stick or brown too quickly, add 1-2 tablespoons water and reduce heat. Add sugar if using in the last 10 minutes to enhance caramelization.
4. Add Thyme: In the final 5 minutes of cooking, stir in fresh thyme. Remove from heat and let cool slightly.

Prepare the Tart:

5. Preheat Oven: Preheat oven to 375°F (190°C).
6. Prepare Crust: Press pie crust or puff pastry into a 9-10 inch tart pan with removable bottom, trimming excess. Prick bottom with fork. Line with parchment paper and fill with pie weights or dried beans.
7. Blind Bake: Bake crust for 15 minutes. Remove weights and parchment, bake 5 more minutes until lightly golden. Remove from oven.
8. Make Custard: In a bowl, whisk together eggs, heavy cream, salt, pepper, and nutmeg if using until well combined.
9. Assemble Tart: Sprinkle half the Gruyère cheese over the bottom of the pre-baked crust. Spread caramelized onions evenly over cheese. Top with remaining cheese. Pour custard mixture over onions, ensuring it fills the gaps.
10. Bake: Bake for 30-35 minutes until custard is set (slight jiggle in center is okay) and top is golden brown.
11. Cool and Serve: Let tart cool for 10-15 minutes before removing from pan. Serve warm or at room temperature.