Place the steaks in the freezer for 30 minutes to chill thoroughly.
Slice the onion into thick slices.
Remove the bison steak from the freezer, and slice into very thin slices.
Heat a cast iron skillet over medium heat and add 1 Tablespoon olive oil. When the oil is hot, add the onions and stir them till soft and beginning to turn brown. When the onions are done, put them on a plate and set aside.
Add some more oil to the pan and turn to med-high heat. Add the bison meat. Spread it out as evenly as possible. Fry the meat quickly on both sides, just until the pink is mostly gone but the juices haven't fried off.
Season with Cavender's greek seasoning. Stir the onions back into the meat, and cover the meat with slices of cheese. Remove skillet from heat and cover skillet with a large lid or piece of foil, and let it sit for 2 minutes to allow the cheese to melt.
While cheese is melting, heat a griddle or other large skillet over med-high heat. Slice 4 rolls and butter generously. Toast the rolls on the griddle. Divide the meat mixture into 4 equal parts and place it in the hot rolls. Serve warm.