Warm 2 tbsp olive oil in a large pan over medium heat. Add in sliced onions and cook until translucent, about 7 minutes. Add in garlic and saute for 30 more seconds. Add in red wine and balsamic vinegar, reduce to medium-low, and simmer until reduced to a jam-like consistency. Add in the juice from half a lemon, salt and pepper to taste, then set aside.
Heat remaining olive oil over medium-high heat. Salt and liberally pepper steak, then cook in pan until desired doneness. Let it rest for a couple minutes, then slice into thin cuts.
To assemble sliders, spread red onion jam onto rolls, top with steak, swiss cheese, and desired garnishments. Enjoy!