Prep the zucchini: Slice zucchini into 1/4-inch half-moons. Toss with 1 teaspoon salt and spread on a sheet pan lined with paper towels. Let sit 15–20 minutes to draw out moisture, then blot dry.

Preheat the oven: Set to 375°F (190°C). Grease a 9x13-inch baking dish.

Cook the sausage: Heat oil in a large skillet over medium. Add sausage and break it up as it browns, 6–8 minutes, until no longer pink. Drain excess fat if needed, leaving about 1 tablespoon in the pan.

Sauté aromatics: Add onion to the skillet and cook 3–4 minutes until softened. Stir in garlic for 30 seconds until fragrant.

Make the creamy tomato sauce: Reduce heat to low. Stir in tomato sauce, heavy cream, cream cheese, Italian seasoning, and red pepper flakes if using. Whisk until smooth and creamy. Season with salt and pepper. Simmer 2–3 minutes.

Combine: Fold the zucchini into the skillet with the sausage and sauce. Mix gently to coat without breaking the zucchini.

Assemble: Transfer mixture to the baking dish. Top evenly with mozzarella and Parmesan.

Bake: Bake 18–22 minutes until the cheese is melted and bubbling. For a golden top, broil for 1–2 minutes at the end, watching closely.

Finish and serve: Rest 5–10 minutes to set. Sprinkle with chopped basil or parsley. Serve warm.