Heat a large non-stick pan over medium heat. Don't separate the buns. Individually remove the tops from the bottoms, creating a top sheet and a bottom sheet. Melt 1/3 of the butter in the pan, take the top sheet of the buns and place into the pan, cook until golden brown then remove and set aside. Do the same with the bottoms of the buns using 1/3 of the butter.
In the same pan, add the remaining butter and the oil. Add in the onions and sauté until golden brown. Then spread out the onions onto the bottom sheet of buns.
Add the broth and beef to the pan, season with salt and pepper. Bring to a low simmer.
Whisk the flour with an equal amount of water to form a smooth paste, use more water if needed so there are no lumps. Slowly stir the flour/water into the simmering pan until the broth thickens.
Taste and adjust the seasoning with salt and pepper. Lay the cheese over top, cover with a lid and melt. Once melted, place over the onions. Cover with the bun top sheet, portion and serve with pickles and chips.