Mushroom sauce: add butter to a large pan over medium heat. Once the butter is fully melted and bubbly, add onions and mushrooms and cook for 5 minutes until they have both softened.
Add the minced garlic, stir, and cook another 2 minutes.
Spoon 2 tablespoons of all-purpose flour into the pot and cook until the flour is no longer raw. About 2 minutes.
After 2 minutes pour in the beef broth and milk and add the salt, pepper, and MSG (if using).
Cook the mushroom sauce for 10 to 15 minutes, stirring often, or until the sauce has thickened to your liking.
Garlic bread: combine the butter, parsley, minced garlic, and parmesan cheese and stir to combine well. Spread this compound butter on the inside of the top and bottom of your hoagie roll.
Place the buttered bread slices under a broiler and broil until the butter is melty and the bread is browning and starting to get toasty.
Steak patty: while the bread is broiling, form the ground steak into a rectangle that's an inch or so wider and longer than the bread. I form my patties on a piece of parchment paper so that they are easier to transfer to the hot pan.
Salt and pepper the top side of the beef patty.
Place a medium pan over medium-high heat. After 3 minutes or so of heating the pan, add your beef patty to the pan, seasoned side down. Season the top of the patty and cook it for 3 minutes or so on the first side. Then flip and cook an additional 3 minutes.
Sandwich assembly: once the patty is fully cooked and the garlic bread is toasted, spread a tablespoon or two of mushrooms and sauce on the bottom bun and then place the ground steak patty on top.
Add more mushrooms and sauce on top of the patty and cover with the shredded cheese. Place the bottom part of the sandwich in the broiler again to melt the cheese.
Once the cheese is melty continue building the sandwich by topping it with pickles and sliced onion and then the top part of the bun.
Serve and enjoy.