Heat the oil in a large skillet over medium heat. Melt butter into oil then add onions and stir to coat in butter. Season with salt, pepper, thyme and bay. Cook onions, stirring occasionally, until caramel in color, 30-40 minutes depending on how thinly you sliced them. Deglaze the onions with sherry and remove the bay leaf; add stock.
Heat rolls in warm oven to crisp them up a bit.
Dip rolls in the onions and stock to wet, and place them on a baking sheet. Dip meat in the broth and fill rolls, top beef with some onions, draining off most the stock; top onions with cheese and sprinkle cheese across the roll tops as well. Broil cheese to melt and serve sandwiches with more broth and a green salad alongside for dipping.