01 - Preheat oven to 400°F. Slice the top off the garlic head, drizzle with 1 tablespoon olive oil, wrap in foil, and roast for 30–35 minutes until soft. Let garlic cool, then squeeze out roasted cloves.
02 - In a mixing bowl, combine flour and salt. Cut in the cold cubed butter until the mixture resembles coarse crumbs. Gradually add ice water, 1 tablespoon at a time, mixing just until the dough comes together. Form into a disk, wrap in plastic, and refrigerate at least 30 minutes.
03 - Heat the remaining tablespoon olive oil in a skillet over medium heat. Sauté the sliced onion for about 5 minutes until soft. Add zucchini and mushrooms and cook for 5–7 minutes until vegetables are tender and any excess liquid evaporates. Season mixture with salt, black pepper, and thyme. Remove from heat.
04 - In a bowl, combine roasted garlic cloves with ricotta and grated Parmesan cheese. Fold in the sautéed vegetables until well integrated.
05 - On a lightly floured surface, roll the chilled dough into a 12-inch (30 cm) circle. Transfer the dough to a parchment-lined baking sheet.
06 - Evenly spread the vegetable and cheese filling over the dough, leaving a 2-inch (5 cm) border. Fold dough edges over the filling, pleating as necessary to create a rustic shape. Brush the exposed crust with beaten egg and sprinkle with extra grated Parmesan cheese.
07 - Bake for 35–40 minutes until the crust is golden brown. Allow the galette to cool for 10 minutes before slicing. Garnish with chopped parsley if desired.