Step 1

Preheat the oven to 350 degrees F. Lightly spray a 9 x 13 inch baking dish with cooking spray.

Step 2

Place a large stockpot filled with at 4 quarts water and 1 Tablespoon salt over high heat. When it comes to a rolling boil, cook noodles for the time specified on the package. Drain.

Step 3

While water heats and pasta cooks, place a 10 or 11 inch x 3 inch deep pot over medium high heat. Add the beef and cook until it is no longer pink, breaking up chunks with the back of a spoon.

Step 4

Use a slotted spoon to remove meat, but leave any fat and juices in the pan. Over medium heat, cook the onions and thyme in the same pan about 2 minutes until onion is translucent. Add mushrooms and continue to cook on medium high heat, stirring until liquids have released from the mushrooms and then evaporated, about 5 minutes.

Step 5

Return meat to the pan, add broth and tomato paste and stir over medium heat 2 minutes.

Step 6

Off heat, stir in sour cream and salt and pepper. Add noodles to the pan and stir, then add meat and about half the mozzarella, stirring just to combine. Pour into prepared bake dish and sprinkle with remaining cheese. Cover with a piece of foil which has been sprayed with cooking spray and bake 25 minutes. Remove foil and continue to bake until lightly browned, about 10 more minutes.