Beef stroganoff: slice the beef into 1/2-inch-wide slices that are around 2 inches long.
Add 2 tablespoons of olive or vegetable oil to a large skillet over medium-high heat. Once the oil starts shimmering, without crowding the pan, place slices into the pan and cook each piece of meat for 1 minute per side. Cook the meat in batches, and when one group gets browned, move them out of the pan and into a plate or bowl to rest. Continue cooking the meat until all of it has browned and is moved to the plate or bowl. The meat does not need to be cooked all the way through; it will cook more later.
Do not clean out the pan, but reduce the heat to medium and then add 3 tablespoons of butter along with the diced onions and sliced mushrooms. Cook for 7 to 8 minutes, stirring often until the mushrooms and onions have softened.
Add the minced garlic, stir, and cook for 1 minute. Keep the garlic moving and mixed in with the onions and mushrooms.
After the garlic has cooked for a minute, add 2 tablespoons of flour and cook it for 1 minute, stirring constantly while the flour cooks.
Add the beef broth and white wine and cook until the liquid starts simmering. Once the liquid is simmering and bubbling, add all of the meat back into the liquid along with any residual liquid from the bowl or plate the meat was resting in. Bring the liquid back to a simmer and cook everything for 20 minutes.
After 20 minutes, the meat should have softened some and the liquid should have reduced a bit. Add the Dijon mustard, 1/2 teaspoon salt, and 1/4 teaspoon ground black pepper.
Add the sour cream and cook for 5 additional minutes to make sure everything is warmed through. If the liquid hasn't reduced and the sauce isn't thick enough, you can continue to simmer for an additional 10 minutes or so to help thicken things up. If you think the sauce is too thick, you can add more beef broth or water, 1/4 of a cup at a time.
Sandwich assembly: slice a crusty roll and toast it if it's more than a day old. To do this, I preheat the oven to 350°F (175°C) and bake the roll for 5 minutes.
Add scoops of beef stroganoff and plenty of sauce/gravy, top everything with some crispy store-bought shoestring potatoes, and garnish. Serve with additional sauce for dipping if desired, and enjoy.