Cook tortellini in boiling salted water for 3 minutes until tender. Reserve 1 cup pasta cooking water before draining.
In a large skillet, melt butter over medium heat and add minced garlic. Cook for 30 seconds until fragrant.
Whisk in flour to form a paste and cook for 1 minute to eliminate raw flour taste.
Gradually whisk in milk, ensuring no lumps form, and bring to a gentle simmer.
Stir in grated Parmesan cheese until melted and smooth.
Season with salt and pepper to taste, adding reserved pasta water as needed to achieve desired consistency.
Add cooked tortellini to the sauce and toss gently to coat.
Garnish with fresh parsley and additional Parmesan cheese before serving.