Make the sauce. You’ll start out by creating a “roux” with the flour and butter. Melt 2 tablespoons of butter on low. Then mix in the flour until it’s all congealed.
Slowly add the milk, with the burner on low to medium-low so you don’t burn the mix. This will take awhile. Keep stirring periodically and keeping a close eye on the sauce. Let it heat and stir until the mix sickens to a mac-and-cheese sauce consistency. Add the white wine. (You may need to add a little extra flour if the wine thins out the mixture too much.) Add the mustard (3 quick squeezes), garlic powder, pepper, salt, red pepper flakes, and Italian seasoning to taste.
Make sure the sauce is really hot, and then add the cheese. This will further thicken the sauce. Stir and stir until the cheese is completely melted into the sauce.
Pour the saute mix into the sauce and mix really well.