Heirloom Tomato, Zucchini, Caramelized Onion and Feta Galette

Ingredients (11)

• 2 yellow onions
• 1 tablespoon olive oil
• 2 tablespoons butter
• 2 refrigerator pie crusts
• 1 egg white
• 2 tablespoons Dijon mustard
• 2 heirloom tomatoes
• 2 zucchini
• 1/2 cup feta cheese
• Sprigs of fresh thyme leaves
• Kosher salt and freshly ground black pepper