In a medium saucepan, melt the butter over medium heat.
Add the flour and whisk until smooth.
Gradually add the milk, whisking constantly until the mixture is smooth and creamy.
Add the shredded cheese, (salt, garlic powder, onion powder, and cayenne pepper are all optional) stirring until the cheese is melted and the sauce is smooth.
Remove from heat and serve immediately on the toasted nachos.
Garnish the tray with salsa and jalapeños.