French Onion Mashed Potato Gratin

Ingredients

• 6 cups (900g) Yellow Onions, peeled, sliced ¼ inch
• 2 tablespoons (28.4g) Unsalted Butter
• 2 teaspoons (2g) Salt
• ½ tablespoon (3g) Freshly Ground Black Pepper
• 1 quart (960ml) Vegetable Stock or Vegetable Broth
• 2 tablespoons (30ml) Worcestershire Sauce (optional)
• ½ teaspoon (~1g) Dried Thyme (Fresh Thyme can be subbed)
• 1-1/3 cup (320ml) Water
• 1 teaspoon (~1g) Salt
• ¼ cup (56.5g) Unsalted Butter, softened
• ¾ cup (180ml) Whole Milk (Any milk can be subbed), cold
• 2-2/3 cup (135g) Instant Mashed Potatoes (Dehydrated Potato Flakes)
• 1/3 cup (80ml) Buttermilk
• ¼ cup (60g) Sour Cream
• 8 ounces (226g) Provolone Cheese Slices
• 4 ounces (113g) Gruyere Cheese, grated
• 1/3 cup (30g) Parmesan Cheese, grated
• As Needed Flakey Sea Salt (such as Maldon)
• 1 teaspoon Fresh Thyme Leaves