Preheat Oven: Preheat to 425°F (220°C).
Slice Cauliflower: Trim leaves and stem, then slice the cauliflower into 1-inch "steaks."
Season: Mix olive oil, garlic powder, smoked paprika, salt, and pepper. Brush on both sides of the steaks.
Bake: Place on a parchment-lined baking sheet and roast for 25-30 minutes, flipping halfway, until golden and tender.
Cook Pasta: Boil fettuccine according to package instructions; drain and set aside.
Sauté Chicken: Melt butter in a skillet over medium heat. Add diced chicken, season with salt and pepper, and cook for 6-8 minutes until browned. Remove and set aside.
Make Sauce: In the same skillet, sauté minced garlic for 1 minute. Add heavy cream, simmer, and stir in Parmesan until melted.
Combine: Add cooked pasta and spinach (if using) to the sauce. Return chicken to the skillet, mixing to coat. Season with Italian seasoning, salt, and pepper.
Plate: Serve the creamy chicken pasta alongside roasted cauliflower steak and garnish with fresh parsley.