1 stick salted or unsalted butter, sliced into a few pieces
2 large head garlic, halved crosswise
Kosher salt
18 ounces (2x 9 ounce bags) baby spinach
1 pound (455 grams) thin spaghetti such as angel hair or capellini
Freshly ground black pepper
Pecorino romano, to finish
Heat oven to 375°F (190°C).

Cut butter into slices, arrange in a small baking dish. Sprinkle with salt. Cut garlic heads in half (no need to peel). Arrnage in butter. Cover dish with foil, bake for 35-45 minutes until garlic is completely soft. (Note, don't use stray cloves, they will burn).

Empty the garlic cloves into the melted butter. I do this by lifting the peels out of the butter with tongs, allowing most cloves to fall out, and using the tip of a knife to free the cloves that don’t. Scrape any browned bits from the sides of the baking vessel into the butter.

Start boiling water.

Place the spinach in a blender, and pour the garlic butter over it. Add several grinds of black pepper, and/or a couple pinches of red-pepper flakes, and blend the mixture until totally smooth. If to thick, add 1 to 2 tablespoons of pasta water (from boiling pot with pasta).

Cook your pasta in well-salted water until 1 to 2 minutes shy of done. Before you drain it, ladle 1 cup pasta water into a cup, and set it aside. Hang on to the pot you cooked the pasta in.

Pour the spinach sauce into the empty spaghetti pot, and add the drained pasta and a splash of pasta water. Cook over medium-high heat, tossing constantly, for 2 minutes, until the sauce thickens and coats the pasta. If the pasta sticks to the bottom of the pot, add more reserved pasta water. Finish with freshly grated cheese.

Originally from: https://smittenkitchen.com/2022/11/green-angel-hair-with-garlic-butter-smitten-kitchen-keepers-is-here/