Cook the pasta shells in salted boiling water until just shy of al dente, about 1 minute less than the package directs.
Drain the pasta in a colander and rinse immediately under cold water until completely cool. Shake off excess water and set aside in a large mixing bowl.
Dice the ham, cheddar cheese, celery, and red onion into uniform ¼-inch pieces. Thaw the peas under cool water and pat them dry.
In a medium bowl, whisk together the mayonnaise, sour cream, Dijon mustard, lemon juice, and fresh dill until smooth. Season with salt and pepper to taste.
Add the diced ham, cheddar, peas, celery, and red onion to the cooled pasta. Pour about three-quarters of the dressing over the top and fold gently to combine.
Cover the bowl and refrigerate for at least 30 minutes to allow the flavors to meld.
Just before serving, fold in the reserved dressing to refresh the texture. Taste and adjust seasoning with salt, pepper, or lemon juice if needed. Serve cold.