Step 1: Cook the Hamburger Mixture

1. In a large skillet over medium heat, cook the ground beef until browned, about 5-7 minutes.
2. Add the diced onion and minced garlic, cooking for another 3-4 minutes until the onions are soft and translucent.
3. Season the beef with salt, pepper, smoked paprika, and thyme.
4. Remove from heat and set aside.

Step 2: Sauté the Mushrooms

1. In a separate pan, melt butter over medium heat.
2. Add the sliced mushrooms and sauté for 5-6 minutes until they are golden brown and tender.
3. Season with salt and pepper, then set half of the mushrooms aside for the topping.

Step 3: Prepare the Creamy Sauce

1. In a mixing bowl, combine 1½ cans of cream of mushroom soup, sour cream, milk or broth, and garlic powder.
2. Stir well until the mixture is smooth and creamy.

Step 4: Assemble the Casserole

1. In a large bowl, combine the cooked rice, ground beef mixture, and half of the sautéed mushrooms.
2. Pour the creamy sauce into the bowl and mix until all ingredients are well-coated.
3. Transfer the mixture to a lightly greased baking dish, spreading it evenly.

Step 5: Add the Topping

1. Spread the reserved ½ can of cream of mushroom soup over the top of the casserole.
2. Sprinkle the remaining sautéed mushrooms evenly over the top.

Step 6: Bake the Casserole

1. Preheat the oven to 350°F (175°C).
2. Cover the dish with foil and bake for 25-30 minutes until the casserole is heated through and bubbly around the edges.
3. Remove the foil during the last 5-10 minutes to let the top brown slightly.

Step 7: Serve

1. Let the casserole rest for 5 minutes before serving.
2. Garnish with fresh parsley, if desired, for a pop of color.