Brush both sides of London broil with olive oil and season with salt and pepper. Place the steak on a broiler pan and broil about 4-inches from the heat source, about 5 minutes per side. Lay the portobellos and onions in a single layer on a sheet pan lined with aluminum foil. Drizzle with oil and vinegar then put in the oven and roast for 10 minutes. Remove the steak to a cutting board. Let rest for 5 minutes then slice the steak thinly against the grain.
In a small bowl, combine mayonnaise, horseradish, parsley, and lemon juice. Spread mayonnaise mixture on both sides of the split ciabatta. Lay several slices of steak on the bread and mound with a bit of mushrooms and onions. Cover with a couple of slices of cheese. Garnish with arugula, and serve.
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