My Famous Homemade Shepherds Pie

Ingredients
For the Mashed Potatoes:

3 1/2 pounds (1.6kg) russet potatoes (about 4 large), peeled and cut into 1-inch pieces
Kosher salt
6 tablespoons (85g) unsalted butter, cubed

For the Meat Sauce:

1 1/2 cups (360ml) homemade or store-bought low-sodium chicken stock
2 packets unflavored gelatin (about 5 teaspoons; 1/2 ounce; 14g)
2 tablespoons (30ml) vegetable oil
2 1/2 pounds (1kg) ground beef or lamb, or some combination (see note)
1 large yellow onion (about 14 ounces; 400g), diced
3 medium carrots (about 8 ounces; 225g), diced
2 ribs celery (about 4 ounces; 110g), diced
2 medium cloves garlic, minced
2 tablespoons (30ml) tomato paste
1 cup (240ml) dry red wine
2 sprigs thyme
1 bay leaf
1 tablespoon (15ml) Worcestershire sauce
1 teaspoon (5ml) Marmite (optional)
2 tablespoons (15g) all-purpose flour
8 ounces (225g) frozen peas
Kosher salt and freshly ground black pepper

1 1/2 cups (360ml) heavy cream
Grated Parmigiano-Reggiano cheese, for topping (optional)