Meatless Mushroom Tart

INGREDIENTS

• 1 (17.25 ounce) package frozen puff pastry, thawed according to package instructions
• 3 tablespoons butter, melted
• 2 1/2 cups chopped yellow onion
• 8 cups portobello mushrooms, coarsely chopped
• 2 tablespoons fresh thyme leaves, finely chopped
• 1 (8 ounce) package cream cheese, cut into cubes, at room temperature
• salt and ground black pepper to taste
• 1 large egg yolk
• 1 tablespoon milk