Melt 2 tablespoons butter in a saucepan over medium heat.
Add about ¾ to 1 cup finely chopped celery and cook for 4–5 minutes until softened.
Stir in flour and cook for 1–2 minutes, stirring constantly. This makes a roux, just a simple flour-and-butter mixture that thickens the soup.
Slowly whisk in 1 cup milk and ½ cup broth (chicken or vegetable).
Continue cooking and stirring until thick and creamy, about 3–5 minutes.
Add ½ teaspoon salt, ¼ teaspoon pepper, ¼ teaspoon garlic powder, and ¼ teaspoon onion powder (adjust to taste).
Cook another minute until smooth.