Line a pan with foil and preheat broiler to high. Arrange peppers on foil-lined baking tray and broil to blister the peppers and char evenly all over, 15 to 20 minutes, turning once.
When they are charred evenly and tender, place the peppers in a bowl and cover to cool peppers.
In a small skillet over medium heat, melt 2 tablespoons butter. When it foams add onions and stir, cook 10 minutes. Smash garlic and add it to the onions. Add about 1/2 cup water and cook 10 to 15 minutes more to light golden and very sweet.
Preheat a large cast-iron skillet over medium-high heat.
Heat a sauce pot over medium to medium-high heat. Melt 4 tablespoons butter and when it foams whisk flour into butter and slowly add the milk. Let sauce come to a bubble and whisk to thicken until sauce can coat a spoon. Stir in cheese to melt, turn off heat and cover.
Place the oil near skillet and a large, thin meat spatula or sturdy tongs. Pat meat dry and ruffle the slices up to loosen them. Combine the salt, pepper and granulated garlic in a small dish then sprinkle over meat. Cook half the meat at a time by squirting oil on to skillet to lightly coat it then cook meat until brown and crispy, 3 to 4 minutes per batch. Pile cooked meat on platter. Add bone broth to skillet, let the drippings bubble up and reduce heat to simmer.
Peel the roasted peppers and seed them. For warm rolls, place in oven once the peppers are removed.
To assemble: Line a roll with a few spoonfuls of soft onions. Dip the beef into broth or stock and fill rolls. Top with roasted peppers, cheese sauce, cherry peppers and giardiniera.