Gather all ingredients.

Combine water and rice in a small saucepan; bring to a boil. Reduce heat to low and simmer until tender, about 20 minutes. Remove from heat and set aside.

Preheat the oven to 325 degrees F (165 degrees C). Cook sausage and onion in a large skillet over medium heat, stirring until evenly browned. Drain excess grease.

Stir in zucchini and tomatoes and cook until tender.

Stir in cooked rice, mushrooms, and cheese. Season with oregano, salt, and pepper.

Spread mixture into a 9x13-inch baking dish.

Bake, uncovered, in the preheated oven until lightly browned and bubbly, about 1 hour.