Tater Tot Casserole Hotdish Recipe

Name: Tater Tot Casserole (Hotdish) Recipe
Ingredients: 5 cups (570g) button mushrooms, washed, dried, and quartered; 2 tablespoons (30ml) extra-virgin olive oil; 12 sprigs thyme, divided; Kosher salt and freshly ground black pepper, to taste; 2 1/4 cups (450g) whole milk; 1/2 onion (125g), peeled and roughly chopped; 2 cloves (8g) garlic, smashed; 2 tablespoons (28g) unsalted butter; 3 tablespoons (25g) flour; 1 tablespoon (15ml) vegetable oil; 1 pound (454g) 90% lean ground beef; 1 small (250g) onion, diced; 2 cloves (8g) garlic, finely minced; 1 tablespoon (8g) all-purpose flour; 3/4 cup (180ml) chicken stock, either homemade or low-sodium store-bought; 3/4 cup (100g) frozen or fresh peas; 3/4 cup (100g) frozen or fresh corn kernels; 1/4 cup (30g) chopped parsley; 1/4 cup (30g) chopped chives; 1 tablespoon (15g) Dijon mustard; 2 teaspoons (10ml) Worcestershire sauce; 1 pound (454g) Tater Tots, frozen or homemade; Ketchup, for serving