Cook pasta according to directions on package. During the last minute of cooking add the peas. Drain and reserve 1/4 cup of the cooking liquid.
In a large skillet over medium heat, melt the butter. Add the shrimp and sprinkle with 1/2 tablespoon of Old Bay. Cook for 3 minutes or until they start to turn pink. Reduce heat to medium-low.
Stir in the half-and-half, reserved cooking liquid, Parmesan cheese and the rest of the Old Bay. Simmer for 5 minutes or until the sauce begins to thicken. Remove from heat and add the pasta, peas and green onion. Let stand for 5 minutes before serving (this gives the sauce a few more minutes to thicken)