Name: Mini Mushroom Tarts
Ingredients: 1 large yellow onion, thinly sliced; 2 Tbsp. olive oil, divided; ¼ tsp. ground black pepper; 16 oz. baby bella mushrooms, sliced; ½ tsp. dried thyme; 4 oz. Gruyere cheese, shredded; 1 large egg, beaten; Chopped fresh parsley for garnish, optional; 2 (10-inch) square sheets frozen puff pastry, thawed; 2 cloves garlic, minced; ¼ tsp. salt; 3 Tbsp. whole grain mustard