In a 12 inch cast iron skillet, melt butter over medium heat. Whisk in flour to make a paste, and cook until bubbly, about three minutes.
Whisk in dried minced onions, milk, and chicken base. Bring to a slow boil over medium-high heat, stirring occasionally.
When the mixture begins to thicken, add kosher salt, freshly ground pepper, and cheese.
Turn the heat off, and stir in ham, corn, and potatoes. Carefully stir the mixture together so that the sauce evenly covers the potatoes.
Transfer the skillet into the preheated oven and bake for 30 minutes.
While the skillet is baking, mix the crushed Ritz crackers, melted butter, dried parsley, salt, and pepper together in a small bowl. Stir well to coat the crumbs in butter.
Remove the skillet from the oven after 30 minutes, sprinkle the Ritz crumbs over the top evenly, and return the skillet to the oven to bake for another 15-20 minutes. The top should be golden brown and bubbly.