To make provolone sauce, melt bacon fat (or butter) in a medium saucepan over medium heat. (If using butter, let the butter to cook for 30 seconds or so to evaporate water. It should bubble up, but don’t let the butter brown.)
Add flour and cook the flour, stirring continuously, for no more than 2 minutes, or until flour turns slightly golden color. (This’s called white roux, which will thicken the cheese sauce and make it nice and smooth.)
Slowly pour in milk into the roux, while whisking the mixture continuously. Once all the milk is added, cook it for another 2-3 minutes, or until the mixture comes to a boil.
Remove from heat and stir in cheese. Whisk until cheese is fully melted. Season with salt and pepper to taste.
To prepare the rolls, slice hoagie rolls ¾ way and smear 1 tablespoon of butter, or mayo inside of each roll.
Heat a large skillet over medium high heat.
Place the rolls cut-side down on hot skillet and toast until nice and golden. Remove from heat and set aside.
To cook the vegetables, heat 1 tablespoon of oil in the same skillet you toasted the roll. When oil is hot, add the diced onion and bell peppers and cook until nice and soft, about 3 minutes. Transfer onto a small plate.
To cook the meat, heat the remaining 1 tablespoon of oil in the same skillet over medium high heat. Add the sliced steaks, garlic, salt and pepper. Cook, stirring frequently, until meat is fully cooked, no more than 2 minutes.
Remove from heat and divide the meat into 4 equal parts in the skillet. Place 2 slices of provolone cheese over each steak piles. Let it melt for a minute or so. Then place toasted hoagie roll on each steak pile. Slide a spatula under the meat and flip the sandwiches one at a time.
Serve immediately with provolone cheese sauce.