Preheat oven to 400 degrees. Lightly butter a 1 1/2- to 2-quart baking dish.
Cook noodles according to the package directions and drain in collander; do not rinse noodles.
While noodles cook, melt butter in a large saucepan over medium heat, add onion and cook, stirring, until softened (about 5 minutes). Stir milk and cornstarch together in a large cup; whisk into saucepan with onion and cook, whisking, until boiling. Whisk in cream and continue to cook, whisking, until thickened, about 2 minutes.
Stir noodles, peas, ham and salt into sauce mixture, season with pepper and scrape into prepared baking dish. Sprinkle pretzels and Cheddar on top and bake until bubbly and golden on top (about 20 minutes).