Preheat oven to 400 degrees F and brush flatbread with olive oil. Set aside.
Slice onions and caramelize with 2 tablespoons of butter and brown sugar in a small pan over medium heat. Set aside.
Slice peppers and add to the same small pan and saute until slightly cooked.
Add steak to a large pan over medium heat. Add worcestershire sauce and salt and pepper to taste.
Melt 2 tbsp butter over medium heat. Add 2 flour and milk. Bring to a boil, stirring consistently. Add cheeses (mozzarella, provolone and gorgonzola) and continue stirring into fully melted and combined into a slightly thick sauce.
Bake flatbread for 3 to 5 minutes.
Spread cheese sauce over flatbread and top with cooked steak, onions and peppers. Bake for 11-12 minutes.
(Optional) combine radish and lemon juice to drizzle over cooked flatbread.