Preheat oven to 475 F degrees.

Separate all of the onion strips from each other and soak them in the buttermilk for 10 minutes minimum.

Coat a baking sheet pan or two with nonstick cooking spray.

In a large mixing bowl, mix together the breading ingredients: Flour, panko, Italian parsley, and salt. Take one handful at a time of onions that were soaking in buttermilk. Toss the onions with the dry mixture and then place them on the baking sheet pan and spread the sliced onions evenly on the pan.

Place the pan on the middle rack of the oven and bake the breaded onion rings until golden brown, almost 30 minutes, giving the onions a toss 2 - 3 times during this baking period. You can also use an Air Fryer or fry them traditionally.

Remove from the oven and set the onions aside to use later.

Reduce the oven temperature to 400 F degrees.

Place the dehydrated porcini mushrooms in a medium-sized bowl and cover them with at least 3 cups of hot, boiling water, and cover with plastic wrap at least 3 cups of hot boiling water to RE-constitute.

Allow them to steep in the hot water/broth for 20 minutes and then strain the liquid through a fine-mesh strainer, reserving at least 1 cup of the mushroom liquid. Set both aside.

Saute' the chopped pancetta until fully cooked for 8 - 10 minutes. Stir occasionally and when nicely caramelized/or browned, transfer to a paper-towel-lined plate. Drain and discard the hot grease and set the pancetta aside.

Add the green beans and blanch in the boiling water until crisp-tender and bright green (for 3 - 5 minutes).

While beans are boiling, fill a very large bowl with ice water.

After the 5 minute blanch in the boiling water, quickly drain the beans in a colander and immediately plunge the beans into a large bowl of ice water to stop the cooking. Drain the beans and set aside.

Chop up the reconstituted porcini mushrooms.

Melt butter in a heavy skillet on a medium-high heat stove burner.

Add the chopped onions and garlic, cook, stirring until softened and the onions are caramelized about 5 minutes.

Add the chopped mushrooms, 1 teaspoon salt and pepper, and cook, stirring occasionally, until the mushrooms begin to lose some of their liquid, about 10 minutes.

Add the thyme, nutmeg, cayenne pepper, and Worcestershire sauce and cook for 2 more minutes.

Sprinkle the flour over the mixture; stir to combine. Cook for 1 minute.

Whisk in the broth and some of the reserved mushroom liquid (add a little at a time and taste as you go) and bring to a boil.

Reduce the heat to medium-low and add cream, parmesan cheese, and Italian parsley. Cook until the mixture thickens, stirring occasionally, for 10 - 12 minutes.

Remove from the heat and stir in all of the blanched green beans and the pancetta.

Transfer everything into a 3-quart baking dish (you can refrigerate this casserole, covered, for up to 4 hours, but keep the onion topping at room temperature).

Bake the casserole until bubbling, about 20 minutes. Then sprinkle with the onion topping.

Bake until warmed through, about another 5 minutes.

Garnish (Optional) Sprinkle minced Italian parsley and diced, sauteed pancetta all over the top of the casserole.