Bring a large pot of water to a boil. Stir in the pasta-water salt. Add the tortellini and cook according to package directions until tender.
While the tortellini cooks, make the sauce: melt the butter in a medium saucepan over medium heat.
Whisk in the flour and cook for 1 minute, whisking constantly, to form a smooth paste (roux).
Slowly pour in the milk while whisking to prevent lumps. Continue whisking and cooking until the sauce thickens, about 3–5 minutes.
Remove the pan from the heat. Add the shredded cheese a handful at a time, stirring until fully melted and smooth.
Season the cheese sauce with garlic powder, sauce salt, and black pepper. Taste and adjust seasoning as needed.
Drain the tortellini well, then add it to the saucepan. Gently toss until every piece is coated in the cheese sauce.
Serve hot. Garnish with chopped parsley if desired. If the sauce thickens too much, loosen with a splash of milk or a spoonful of reserved pasta water.