Tater Tot Casserole

Instructions

1. For the Mushroom Sauce: Adjust oven rack to center position and preheat oven to 350°F (180°C). In a large bowl, toss mushrooms with olive oil and 6 sprigs thyme, then season with kosher salt and freshly ground black pepper. Spread seasoned mushrooms on a foil-lined rimmed baking sheet. Bake until deeply browned, about 45 minutes. Remove from oven and discard thyme sprigs. Increase oven temperature to 375°F (190°C).
2. In a medium saucepot, combine roasted mushrooms with 6 remaining thyme sprigs, milk, onion, and garlic and bring to a boil over medium-high. Remove from heat and steep, covered, for 1 hour. Strain, pressing gently on the solids, reserving liquid and discarding solids.
3. Wipe out pot and return to stovetop. Melt butter over medium heat until foaming subsides, then add flour and mix into a paste. Continue cooking, stirring constantly, until raw flour smell is gone, about 3 minutes. Whisking constantly, add mushroom-infused milk in a thin, steady stream, or in increments of a couple of tablespoons at a time, whisking thoroughly and getting into all corners of the pot to maintain a homogeneous texture. Sauce will initially become very thick, then get very thin once all milk is added.
4. Heat, stirring constantly, until sauce comes to a simmer and begins to thicken slightly, about 5 minutes. Reduce heat to low and cook, stirring often, until sauce is thick enough to coat the back of a wooden spoon, about 3 minutes. Season to taste with kosher salt and freshly ground black pepper. Transfer sauce to a small bowl and place plastic wrap directly on the surface to prevent a skin from forming.
5. For the Ground Beef: In a large skillet, heat oil over medium-high heat. Add ground beef and break up into crumbles with a whisk or potato masher. Cook, stirring occasionally, until well browned, about 8 minutes. Drain off any excess fat, if necessary, leaving only about 1 tablespoon (15ml) in skillet along with meat.
6. Reduce heat to medium, add onion and garlic to ground meat, and cook, stirring often, until onions soften, about 3 to 5 minutes. Add flour and cook, stirring constantly, for an additional minute, until meat mixture is well coated. Stir in chicken stock and simmer until thickened, about 3 minutes.
7. Remove from heat and stir peas, corn, parsley, chives, Dijon, and Worcestershire into ground beef mixture. Season to taste with kosher salt and freshly ground black pepper. Scrape meat mixture into a 2-quart (7 x 11-inch) baking dish.
8. To Finish: Top meat with mushroom sauce, spreading sauce into an even layer, and arrange Tater Tots on top. Bake for 35 to 45 minutes, or until Tater Tots are crispy and golden brown. Serve with squiggles of ketchup.