HEAT oil in large skillet over medium-high heat. Add onions and cook 5 minutes, until translucent. Reduce heat to medium-low and continue cooking, stirring occasionally, until onions begin to caramelize, about 40 minutes.
HEAT grill or grill pan over medium-high heat.
GRILL naan 2 to 3 minutes per side, until grill marks appear. Remove from heat and set aside.
COMBINE horseradish, sour cream, mayonnaise, parmesan and thyme in a small bowl. Season with salt and pepper to taste.
SPREAD horseradish sauce over warm flatbreads.
TOP with caramelized onions, roast beef and arugula.
GARNISH with additional parmesan and black pepper, and serve!