Preheat and prep. Heat your oven to 375°F (190°C). Grease a 9x13-inch baking dish with olive oil or butter.
Brown the sausage. In a large skillet over medium heat, cook the sausage, breaking it into crumbles.
Cook until well browned and no pink remains, about 6–8 minutes. Remove to a plate, leaving a bit of fat in the pan.
Sauté aromatics. Add onion (if using) and bell pepper to the skillet with a pinch of salt. Cook 3–4 minutes until softened.
Stir in garlic and smoked paprika; cook 30 seconds until fragrant.
Soften the cauliflower. Add cauliflower florets to the skillet with 2–3 tablespoons of water. Cover and steam-sauté 4–5 minutes until just tender-crisp. Uncover and cook off excess moisture.
Wilt the greens. Stir in chopped spinach and cook 1 minute until just wilted.
Return the sausage to the pan. Taste and season with salt, pepper, and red pepper flakes if you like heat.
Make the creamy base. In a bowl, whisk eggs, heavy cream, cream cheese, Dijon, 1 cup shredded cheese, and a pinch of salt and pepper. Whisk until mostly smooth (small cream cheese bits are fine).
Assemble. Spread the sausage and veggie mixture evenly in the baking dish.
Pour the creamy egg mixture over the top. Sprinkle the remaining 1 cup of cheese evenly.
Bake. Bake 25–30 minutes, until the center is set and the top is bubbly and golden. If needed, broil 1–2 minutes at the end for extra color.
Rest and serve. Let the casserole rest 8–10 minutes so it slices cleanly.
Garnish with chopped parsley and serve warm.