Green Bean Casserole Galette

Ingredients

• Dough
• 3 cups all-purpose flour, plus more for rolling
• 1¼ teaspoons kosher salt
• 1 teaspoon granulated sugar
• 1½ sticks unsalted butter, chilled and cubed
• 1 large egg, beaten
• Filling
• 1 stick unsalted butter
• 1 medium sweet onion, thinly sliced 1½ pounds mixed mushrooms
• 5 garlic cloves, thinly sliced
• Kosher salt and freshly ground black pepper
• 1 cup heavy cream
• 2 sprigs fresh thyme, plus leaves for garnish
• 1 sprig fresh rosemary
• 1 pound green beans, trimmed
• 1 teaspoon lemon zest, plus more for garnish
• Fried Onions
• 1 shallot, thinly sliced into rings
• 2 tablespoons all-purpose flour
• Pinch of kosher salt
• Vegetable oil, for frying