For the tart
In a sauté pan, heat the olive oil and sweat the onions until translucent and limp, about 10 minutes.
Season the onions with salt raise the temperature and cook while stiring until the onions start to brown slightly. Remove from the pan mix in the thyme and let cool to room temperature. Combine the eggs, cream and parmesan in a bowl season with salt and pepper and whisk them together.
Spread the cooled onions in the prebaked tart shell in an even layer. Pour the cream mixture over the onions.
Bake at 190⁰C until a knife blade inserted in the center comes out clean, 40 to 45 minutes. If the pastry begins to overbrown, cover the edges with aluminum foil. Remove the tarts from the oven and cool on racks. Let the tarts rest for at least 45 minutes before cutting. Serve hot, warm or at room temperature.