Bring a 4-quart / 3.8 L pot of generously salted water to a boil. Cook 340 g / 12 oz fettuccine according to package directions until al dente, about 9 minutes. Scoop out 1 cup / 240 ml pasta water, then drain the pasta.

Pat 340 g / 12 oz chicken cubes dry and season with 1/2 tsp salt and 1/4 tsp black pepper. Heat 1 tbsp olive oil in a 30 cm / 12-inch cast-iron skillet over medium-high heat.

Add the chicken in a single layer and sear for 6 to 7 minutes, turning once, until golden and the internal temperature reads 165 F / 74 C. Transfer to a plate.

In a blender or with an immersion blender, blend the 340 g / 1 1/2 cups cottage cheese with 120 ml / 1/2 cup whole milk until completely smooth, about 1 minute. This step keeps the sauce silky instead of grainy.

Lower the heat to medium and melt 1 tbsp butter in the same skillet. Add 3 cloves minced garlic and cook for 30 seconds until fragrant, not browned.

Pour in the blended cottage cheese mixture and bring to a gentle simmer, stirring often, for 2 to 3 minutes. Do not let it boil hard or the sauce can break and turn grainy.

Stir in 40 g / 1/2 cup grated Parmesan until melted, then thin the sauce with reserved pasta water, a few tablespoons at a time, until it coats the back of a spoon.

Add 60 g / 2 cups baby spinach to the skillet and stir until just wilted, about 1 minute.

Return the cooked pasta and seared chicken to the skillet. Toss for 1 to 2 minutes until everything is coated and warmed through.

Season with salt, black pepper, and 1/2 tsp red pepper flakes if using. Serve right away with extra Parmesan and chopped parsley.