Bring a large pot of salted water to a rolling boil. Cook pasta to al dente according to package directions. Reserve 1/2 cup pasta water before draining.
While pasta cooks, heat a large skillet over medium-high. Add a drizzle of olive oil, then sausage. Break into crumbles and cook until browned and cooked through, about 6–8 minutes. Transfer to a plate, leaving drippings in the skillet.
Lower heat to medium. Add remaining olive oil if needed, then onion. Cook until translucent and lightly golden, 3–4 minutes. Stir in garlic, red pepper flakes, and Italian seasoning; cook 30 seconds until fragrant (nose says yes).
Pour in white wine or broth. Scrape up browned bits from the pan—this is flavor gold. Simmer 1–2 minutes to reduce slightly.
Add crushed or diced tomatoes and return sausage to the skillet. Simmer 5–7 minutes, stirring occasionally, until the sauce thickens and tastes cohesive. Season with salt and pepper.
Stir in spinach, letting it wilt. Add heavy cream (if using) and warm through. If the sauce feels too thick, splash in reserved pasta water.
Add drained pasta to the skillet. Toss with the sauce, adding pasta water as needed to make it silky. Stir in Parmesan and the butter to finish. Taste and adjust seasoning—more pepper? A pinch of salt? Do it.
Plate with extra Parmesan and a sprinkle of chopped basil or parsley. Optional: a drizzle of good olive oil for shine. Then accept compliments like you trained for it.