Bring a large pot of salted water to a boil. Cook pasta until al dente according to package directions. Reserve 1 cup pasta water, then drain pasta.

While the pasta cooks, heat a skillet over medium. If using bacon, cook until crisp; transfer to a paper towel and reserve the fat in the pan (or wipe pan and use 1 tbsp butter for a meatless version).

In the same skillet, melt butter and sauté garlic briefly (20–30 seconds) until fragrant. Add spinach and cook until wilted (about 2 minutes).

Transfer wilted spinach to a blender with 1/2 cup cream. Puree until smooth and bright green. If the mixture is too thick, add another tablespoon of cream or a splash of pasta water.

Return the pureed spinach-cream to the skillet over low heat. Stir in grated cheddar a handful at a time until melted and incorporated. Add peas now if using.

Toss drained pasta into the sauce. Add reserved pasta water, a few tablespoons at a time, until sauce clings to the pasta—aim for glossy, not soupy.

Finish with lemon zest, salt, pepper, and a pinch of nutmeg if using. Sprinkle crisp bacon or toasted breadcrumbs and parsley on top. Serve immediately.