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