Melt butter in a large saucepan over medium-low heat; add garlic and sauté until golden, about 1 minute.
Reduce heat to low and gradually stir in flour until smooth, 3 to 5 minutes.
Add Parmesan cheese and stir until melted and mixture is thickened, 2 to 3 minutes.
Slowly whisk in milk, stirring constantly, until smooth, about 5 minutes.
Season to taste with Italian seasoning, salt, and pepper. Cook and stir until sauce reaches desired consistency, 5 to 10 minutes.
