Start by sautéing sliced onions and sliced garlic in olive oil. When they are nicely browned, deglaze with red wine, and balsamic vinegar and reduce until it’s thick like a jam. Add a bit of Worcestershire sauce, lemon juice, salt, and pepper, and set aside.
Next, season a steak with salt and pepper, and cook it on high heat in a pan, to the doneness that you like. Let it rest before slicing, then assemble your sliders with slices of steak, Gruyere, your onion jam, and mayonnaise.