To begin, start a pot of water to blanch your green beans.
Meanwhile, dice the onion and mince the garlic. Set those aside for a moment.
In a saucepan, sauté the Prosciutto in 2 tablespoons of the butter. When the Prosciutto is golden, scoop it out with a slotted spoon, leaving the drippings in the pot. Toss in the onions & garlic and sauté until just tender. Add in the remaining 2 tablespoons of butter and let it melt. Sprinkle in the flour. Stir it together and let it cook for just a minute. Pour in the whole milk and whisk together.
Add the salt, black pepper, and cayenne pepper. Stir this together and cook over medium heat until it comes to a boil, stirring continually. Let it bubble for one minute and turn off the heat.
Add in the grated extra sharp cheddar cheese. Stir until it is melted in. Set it aside for a moment.
Blanch the green beans. If fresh, blanch for 3 minutes. If frozen, make it 4 minutes. Immediately drain them and run under cold water, or plunge into a bowl of ice water.
Stir the beans, milk sauce, and Prosciutto together in a mixing bowl. Pour the mixture into a casserole dish. Sprinkle the panko bread crumbs over the top.
Bake at 350 degrees for 30 minutes or until the top is golden and the sauce is bubbly.