Prepare pasta according to package directions. Drain and set aside, reserving some of the pasta water.

In a large skillet, melt the butter over medium heat. Saute the garlic and red pepper flakes until golden, about 3 minutes. Add the lemon juice and stir. Turn the heat off.

Add the pasta and pasta water to the pan. Toss to coat. Stir in the spinach and salt. Taste and reseason to taste.

Serve hot with additional butter if desired.