Preheat oven to 350 degrees then coat 9" x 13" casserole dish with non-stick spray.
Break cauliflower into small florets and place in food processor. Pulse until cauliflower pieces are about the size of grains of rice. Measure to make sure you have at least 4 cups of chopped cauliflower. It’s okay to have a bit more.
In a large pot of boiling salted water, cook the cauliflower until just tender, about 3 minutes. Remove from heat and drain then set aside. When cool enough to handle place large handfuls of cauliflower in a clean kitchen towel and wring out as much water as possible. Repeat with remaining cauliflower.
While cauliflower is cooling, place large skillet over medium-high heat then add ground beef. Cook 5–7 minutes or until no pink remains, breaking meat into small pieces as it cooks.
Add mushrooms, onion and garlic then cook 4–5 minutes until the onion becomes translucent and mushrooms soften.
Drain excess grease from skillet, return to stove then reduce heat to medium. Sprinkle flour over meat mixture then mix until flour has dissolved completely. Cook 2 minutes, stirring frequently then add beef broth. Stir to combine, scraping up any brown bits stuck to the bottom of the pan.
Bring liquid to boil then add thyme, paprika, oregano, salt and pepper. Reduce heat to medium-low then simmer for about 5 minutes. Stir in milk, return to boil then reduce heat once again and simmer 5–6 minutes or until the sauce thickens slightly.
Add Parmesan cheese and prepared cauliflower to the beef. Stir to combine then adjust seasoning with salt and pepper if necessary. Transfer mixture to prepared casserole dish then top with more Parmesan cheese.
Bake uncovered 20–25 minutes or until the center is hot and the sides are nice and bubbly. Let casserole cool for 10 minutes before serving.
Make this recipe yours by mixing in a cup for green chilies, jalapeños or bell peppers. Yum!