Fill a large pot with water and add enough salt to make the water taste lightly seasoned. Set the pot over high heat and bring the water to a full boil.

Add the pasta to the boiling water in the pot and stir for a few seconds so it does not stick. Cook it according to the package directions until it is tender with a little firmness in the center.

Zest the lemon into a small bowl and squeeze the juice into another. Slice the garlic cloves into thin chips .

In a large skillet or wide pan, combine the olive oil and 2 tablespoons of the butter over medium heat.

Add the sliced garlic, drained capers, and red pepper flakes to the skillet. Cook for 2 to 3 minutes, stirring often, until the garlic is fragrant and just beginning to turn golden at the edges.

Before you drain the pasta, carefully dip a measuring cup into the pot to reserve 1 cup of the cloudy, starchy pasta water.

Drain the pasta in a colander and immediately transfer the hot noodles into the skillet with the garlic and capers.

Add the lemon juice, the remaining 2 tablespoons of butter, and half of the reserved pasta water to the skillet.

Add the baby spinach to the pan. Toss everything together over medium heat for 1 to 2 minutes until the spinach wilts and the liquid thickens into a smooth sauce that coats the noodles.

Remove the pan from the heat. Stir in the lemon zest and grated Parmesan cheese, adding a splash more pasta water if the sauce looks too dry and serve immediately.