Prepare the Pastry:

In a large bowl, mix the flour and salt. Add the butter and use a pastry blender or fingers to incorporate until the mixture resembles coarse crumbs.
Gradually add ice water, stirring until the dough comes together. Form into a disk, wrap in plastic, and chill for at least 1 hour.

Make the Ratatouille Filling:

Heat olive oil in a large skillet over medium heat. Sauté onions and garlic until translucent.
Add eggplant, zucchini, yellow squash, and bell pepper. Cook, stirring occasionally, until the vegetables are tender.
Stir in tomatoes and thyme, and cook for another 5-7 minutes. Season with salt and pepper. Let the mixture cool.

Assemble the Galette:

Preheat the oven to 400°F (200°C).
On a floured surface, roll out the dough into a 12-inch circle. Transfer to a lined baking sheet.
Spread the ratatouille filling over the dough, leaving a 2-inch border. Sprinkle with Gruyère cheese if using.
Fold the edges of the dough over the filling, pleating to hold it in.

Bake:

Bake the galette for 30-35 minutes or until the crust is golden brown.
Let it cool for a few minutes before serving.

Serve:

Slice and serve the galette warm or at room temperature.