Prep the spinach. Rinse and dry well.
If using regular spinach, remove tough stems and tear large leaves.
Warm the oil. Set a large skillet over medium heat. Add olive oil and let it shimmer without smoking.
Sauté the garlic. Add the sliced garlic and cook 30–60 seconds until fragrant and lightly golden. Don’t brown it deeply or it will turn bitter.
Add the spinach in batches. Pile in a large handful, toss with tongs until wilted, then add more. Keep the heat at medium to medium-high.
Season. Sprinkle in salt, pepper, and red pepper flakes if using.
Toss to distribute the seasoning.
Finish with richness. Stir in the butter if using and cook just until melted and glossy, 15–20 seconds.
Add brightness. Turn off the heat. Add lemon juice or zest and toss. Taste and adjust salt and pepper.
Top with parmesan. Sprinkle the grated parmesan over the warm spinach and toss lightly so it melts into little pockets.
Serve immediately with extra cheese on the side.