In a large dutch oven, melt the butter over medium heat. Whisk in the flour until a 'roux forms. Continue cooking, whisking occasionally, for 1-2 minutes- to let the flour taste cook out.
1/4 cup butter, 1/3 cup flour
Whisk in the half & half, about 1/2 cup at a time, until the mixture's smooth without any remaining lumps, and all of the half & half has been used. Whisk in all of the seasonings. Bring the mixture to a simmer, and allow it to cook stirring occasionally for 6-8 minutes.
2 1/2 cup half & half, salt & pepper, 1/4 tsp smoked paprika, 1 tsp onion powder, 1 tbsp dried mustard, pinch cayenne pepper
Remove the pot from heat and stir in all of the cheese. Continue stirring until the cheese is completely melted/incorporated and the sauce is once again smooth. If using, add the green dye until the desired look is achieved.
8 oz sharp white cheddar cheese, green food coloring
Stir in the spinach, and allow it to wilt in the mixture, about 1 minute.
2-3 cups baby spinach
Add the cooked pasta to the pot and toss it with the sauce until evenly coated.
1 lb radiatore pasta
Serve immediately, and enjoy!