Preheat oven to 400ºF. Bring a large pot of salted water to boil. Add pasta and cook until just tender, about 8 minutes. Drain and set aside.
Melt 3 Tbsp. butter in a large ovenproof skillet over medium heat. Whisk in flour, mustard and 1/2 tsp. salt until smooth. Slowly whisk in milk and half-and-half and bring just to a simmer. Stir in cheese, a little at a time, until melted. Remove pan from heat and stir in peas, ham and pasta. Stir to coat with sauce.
Sprinkle cracker crumbs over top of pasta and dot with remaining 1 Tbsp. butter. Bake until bubbling, about 20 minutes.
For a basic casserole, this was very good. It was enjoyed by parents and child alike. I do wish it had more of a flavor punch and would probably saute onions in the butter before adding the flour. I might also throw in some dried sage or thyme. You can read my full review at Taking On Magazines: http://bit.ly/13zKBYz