Coat the portobello mushrooms in oil, salt and black pepper and place on a sheet pan. Roast in the oven for 15 minutes or until fully cooked. Chill and thinly slice.
Working on a cutting board, spread out a piece of the sliced flank steak. Place a thin line of spinach across the edge of the flank meat. Repeat with the sliced portobello mushrooms and sweet and sour onions. Firmly roll the ingredients up in the flank steak. Secure with a toothpick. Repeat until all the flank meat is stuffed and rolled. Coat a large saute pan with cooking oil and place over high heat. Season each flank steak roulade with salt and pepper. Sear each roulade in the pan by placing the seam side down to ensure closure. Cook until browned on all sides and cooked to desired doneness, about 6 minutes. Slice each roulade on the bias into 1-inch pieces. Serve over arugula and sweet and sour red onions.
For the onions: In a saute pan over medium heat, add the cooking oil and sliced onions. Cook for a few moments while stirring, until the onions start to turn translucent. Add the sugar and salt and continue to cook until onions begin to soften, but are still firm to the touch. Add the red wine vinegar and cook until almost all the vinegar has been reduced. Turn the onions out on to a sheet pan to cool.
This recipe was provided by a chef, restaurant or culinary professional. It has not been tested for home use.