Follow these step-by-step instructions to create a delectable chanterelle mushroom tart:

Step 1: Prepare the Pastry

1. Combine the flour and salt in a food processor. Add the chilled butter and pulse until the mixture resembles coarse crumbs.
2. Add ice water gradually, one tablespoon at a time, and pulse until the dough begins to come together.
3. Transfer the dough onto a floured surface and shape it into a disk. Wrap it in plastic wrap and refrigerate for at least 30 minutes.
4. Preheat your oven to 375°F (190°C).

Step 2: Cook the Chanterelle Filling

5. Heat olive oil in a large skillet over medium heat. Add the onions and garlic, sautéing until the onions are translucent.
6. Add the chanterelle mushrooms to the skillet and cook until they release their moisture and become golden brown, around 8-10 minutes.
7. Stir in thyme leaves, and season with salt and pepper. Remove from heat and let cool slightly.

Step 3: Assemble the Tart

8. Roll out the dough on a floured surface to fit your tart pan. Carefully transfer the dough to the pan and press it into the edges. Trim any excess dough.
9. Prick the base of the pastry with a fork and place a sheet of parchment paper over it. Fill with baking weights or dried beans.
10. Blind bake the pastry in the preheated oven for about 15 minutes. Remove the weights and parchment paper, and bake for an additional 5 minutes until lightly golden.

Step 4: Prepare the Filling

11. In a mixing bowl, whisk together the eggs and heavy cream until smooth. Add the grated Gruyère cheese and mix well.
12. Spread the mushroom mixture evenly over the pre-baked tart shell.
13. Pour the egg and cream mixture over the mushrooms, ensuring even coverage.

Step 5: Bake the Tart

14. Place the tart in the oven and bake for 25-30 minutes, or until the filling is set and the top is golden brown.
15. Remove the tart from the oven and let it cool slightly before serving.