Bring a pot of lightly salted water to a boil. Preheat oven to 375 degrees F. Spray a 9 x 13″ baking dish with cooking spray, and set aside.
Cook pasta according to package directions, and drain.
While pasta cooks, melt butter in a large skillet over medium high heat. Add flour to melted butter, and whisk until combined.
Whisk milk into flour mixture about a half cup at a time, making sure the consistency is smooth before adding more liquid. Bring this sauce base to a low simmer.
Stir cheese, ham, and peas into sauce until cheese is melted. (The peas can be added while still frozen.) Stir in cooked pasta until combined well, and transfer to prepared baking dish.
Melt butter in a microwave-safe dish in the microwave (or on the stove if you prefer). Stir breadcrumbs into the melted butter.
Sprinkle 1/2 cup cheese over the pasta in the baking dish, and sprinkle the breadcrumbs over the cheese. Bake in preheated oven for 20 minutes or until top is golden brown and edges are bubbly. Serve right away.