Prep the spinach. If using fresh, rinse and pat dry. Remove tough stems. If using frozen, thaw fully and squeeze out as much water as possible with clean hands or a towel.
Warm the pan. Set a large skillet over medium heat.
Add butter (and olive oil if using). When it melts and shimmers, add the garlic.
Bloom the garlic. Cook the minced garlic for 30–60 seconds, stirring often. Do not let it brown. You just want it fragrant.
Wilt the spinach. Add spinach in batches if needed.
Toss with tongs until just wilted, 2–3 minutes for fresh. For frozen, stir until heated through and any extra moisture cooks off.
Add the cream. Pour in the heavy cream and stir. Let it bubble gently for 2–3 minutes so it reduces and thickens slightly.
Make it creamy. Stir in Parmesan and cream cheese.
Keep stirring until smooth and glossy. Adjust heat to low so it doesn’t split.
Season. Add nutmeg, red pepper flakes, salt, and black pepper. Taste and adjust.
A tiny squeeze of lemon or a pinch of zest lifts the richness.
Finish and serve. When the sauce clings to the spinach and looks silky, remove from heat. Serve warm beside your protein of choice.