1. Preheat the oven to 400 degrees Fahrenheit.
2. In a hot skillet, add 1 tablespoon of olive oil and the sliced potatoes and onions and brown.
3. Once caramelized, deglaze by adding the red wine vinegar into the skillet and placing into the oven for 10-15 minutes.
4. In the meanwhile, season the sirloin medallions with salt and pepper on both sides.
5. Heat a skillet and add a tablespoon of olive oil. Add the seasoned sirloin into the pan and brown for 2-3 minutes on each side.
6. Add the crumbled blue cheese over the browned sirloin and pop the skillet into the oven for 8-10 minutes.
7. Remove both the skillets from the oven and season the potatoes with salt and pepper.
8. Plate the sirloin and blue cheese along with the caramelized onions and potatoes and serve as main course.