Preheat the oven to 350 degrees F. Line a large baking sheet or 9x13-inch casserole dish with foil and spray the bottom with cooking spray.
PREPARE THE CHEESEBURGER MIXTURE:
Add the beef and onion to a large skillet over high heat. Cook until the meat is browned, about 5 minutes, stirring occasionally to break up the meat. Turn the heat down to medium. Add the garlic and cook 30 seconds more, and then add the ketchup, mustard, Montreal steak seasoning, and Worcestershire sauce. Cook 2 minutes, stirring constantly.
ASSEMBLE THE SLIDERS:
As 1 unit, slice the rolls in half horizontally. Place the bottom half of rolls on the prepared tray. If you do not have a package of slider rolls where all the rolls are stuck together (like King's Hawaiian rolls), then just place all the bottom buns on the baking sheet. Spread 6 slices of cheddar out onto the rolls. Evenly spread the meat mixture on top. Spread the remaining 6 slices of cheddar out on top of the meat. Place the top half of rolls on the cheese.
In a small bowl, mix together the olive oil and melted butter. Brush this on top of the rolls, and then sprinkle on the sesame seeds.
Cover the rolls with foil and bake until they’re warm throughout and the cheese is melted, about 20 minutes. If needed, broil after baking to brown the tops. Serve warm.