For the jam: Burn the onions on a grill or under the broiler until completely charred, about 8 minutes. Chop the burnt onions into a fine paste adding the vinegar in small amounts.
For the steak: Preheat the grill. Butterfly the steak and sprinkle one side liberally with salt and pepper. Thinly coat the same side of the steak with the mustard. Lightly top the mustard with 1 tablespoon of the burnt onion jam, spreading evenly but trying not to mix the two. Sprinkle the parsley and rosemary on top and cover everything with the cheese.
Roll the roulade tightly and secure with three pieces of wet butcher twine. Brush the outside with olive oil, then sprinkle with salt and pepper. Grill for about 12 minutes. Rest the meat, and then cut into four slices and serve with some roasted piquillo peppers.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.