Peas & Cheese Tortellini

I originally got this recipe from Melanie Bauer & it was huge hit in our house, especially with the little ones. 

1 pound cheese tortellini
1 cup frozen peas, thawed
2 tablespoons butter
5 scallions, thinly sliced (white + green parts separated) - *I have used a regular onion before & it tasted just as good.
2 tablespoons flour
1 2/3 cups milk
1/3 cup grated parmesan cheese

In a large saucepan of boiling water, cook tortellini and drain. Meanwhile, add peas butter and scallions to another large saucepan and cook over medium heat for two minutes.  Whisk in flour and cook, stirring, for one minute.  Gradually whisk in milk and bring to a boil, stirring constantly.  Stir in the scallion greens and season with salt and pepper. Stir tortellini and cheese into the sauce.