Prep the cauliflower. Rinse, core, and break into small, even florets. Smaller florets cook faster and absorb more sauce, so aim for bite-size pieces.
Warm the pan. Heat a large, deep skillet over medium heat.
Add olive oil and butter. Once butter melts, add the onion with a pinch of salt. Cook 3–4 minutes until softened and translucent.
Sauté the aromatics.
Stir in garlic, sun-dried tomatoes, Italian seasoning, and red pepper flakes if using. Cook 30–60 seconds until fragrant. Don’t brown the garlic, just wake it up.
Add the cauliflower. Toss florets in the seasoned oil for 2 minutes, letting edges pick up a little color.
Season with 1/2 teaspoon salt and a few grinds of pepper.
Simmer to tender. Pour in the broth and bring to a lively simmer. Cover and cook 5–7 minutes, stirring once or twice, until cauliflower is crisp-tender.
If the pan looks dry, add a splash more broth.
Make it creamy. Reduce heat to medium-low. Stir in the cream.
Simmer uncovered 3–5 minutes, stirring gently, until the sauce thickens enough to coat the back of a spoon. Keep it at a gentle simmer to prevent curdling.
Finish with greens and cheese. Fold in spinach and cook 1–2 minutes until wilted. Sprinkle in parmesan gradually, stirring until smooth and silky.
Add lemon juice. Taste and adjust salt and pepper.
Serve. Top with extra parmesan and fresh basil or parsley.
Spoon over pasta or grains, or serve with good bread to catch the sauce.