Generously season the beef with salt and pepper and form into 2 (1/2-inch thick) oval patties. Cook's Note: You want your patties thin and roughly the same size of the rye bread.

Set aside, or place in the refrigerator while you cook the onions.

Add 1 tablespoon olive oil into a large nonstick skillet over medium-high heat. Once oil is hot, add onion and saute until browned, about 8 minutes.

Season the onions with red pepper flakes, salt, pepper, and chopped parsley. Remove onions from the skillet to a bowl and reserve.

Add the remaining tablespoon oil to the hot skillet. Once hot, add the burgers and cook for 3 to 4 minutes until very browned on one side. Flip and continue cooking the other side for another 3 minutes. Remove from pan.

Preheat a grill pan to medium-high heat.

Assemble the patty melts:

Top 2 pieces rye bread each with a slice of cheese, onions, the burger patty, a second slice of cheese, and a final piece of bread.

Butter both the top and bottom of the sandwich.

Grill each sandwich until golden and the cheese melts, about 3 to4 minutes on each side.

Serve hot with mustard and a side of pickles.

Cook's Note: Ground chuck is cut from the shoulder, ranges from 15 to 20 percent fat, and has a rich flavor that is tender and moist. This is best choice for burgers.