Slice the sirloin steak into 1/4-inch strips. Place in bowl or in a zip-top baggie with the seasoned salt and the vinegar. Stir or shake to cover. Set aside.
Heat oven to 350 degrees. Spread butter on both sides of sliced baguette and place in oven for about 7-10 minutes. Remove from oven when butter is melted and bread is just beginning to toast.
Heat oil in a medium sauce pan over medium heat. Add onion. Cook for about 8-minutes until soft and brown. Sprinkle with salt and pepper to taste. Remove from pan and set aside.
Add walnuts to pan and toast over medium heat. Remove from pan and set aside.
Add steak to pan over medium-high heat – depending on size of pan you may need to cook in 2 batches. Cook until seared on all sides. This should only take about 3 to 4 minutes. Remove from heat. (Repeat if you are doing this in batches).
Slice romaine n half lengthwise, Then cut each half into about 1/2 inch slices. Heap lettuce on 2 plates.
Arrange sliced tomatoes and walnuts around the outside of the plate.
Top each lettuce mound with half of the steak; half of the onions; and then sprinkle with blue cheese (if desired).
Drizzle with dressing and any remaining steak juices from the pan. Arrange bread on the plate. Serve.