Heat 1-2 table spoon of oil in a sauté pan. Add onion rings, cook stirring occasionally until tender about 8 minutes. Add 1 teaspoon of butter and 1 teaspoon of brown sugar. Stir and cook for 1-2 minutes, until the onion is caramelized. Remove from the heat and set aside. Preheat oven to 400 F Season the steak with salt and pepper. Heat approximately one tablespoon of olive oil in a grill pan over high heat. Add the steak and sear on one side for about 2 minutes. Turn and and sear the other side briefly (another minute or so). Place in the oven and continue cooking for about 3 minutes or until the steak is cooked to medium rare level. When the steak is done, remove from the oven. Cover tightly with aluminum foil, and allow to sit for 10 minutes. Slice the steak into strips. Slice the french roll and spread the rest of the butter and heat on a skillet (cut side down). To assemble the sandwiches, spread a tablespoon of mustard on the bottom half of each bun. Place a layer of onion rings, and top with the spinach. Then place the steak slices and pickled jalapeño rings over the spinach, cover the sandwiches with the top half of the buns. Serve with ketchup mixed with Sriracha sauce.