Heat the oven to 350°F (177°C). Butter a 13×18-inch rimmed metal half-sheet pan.
Heat the olive oil in a large skillet over medium-high heat. Add the onion and ground beef. Break the beef into small crumbles and season with salt, black pepper, and garlic powder. Cook for approximately 7–10 minutes, until cooked through and the center reaches 160°F (71°C).
Remove the skillet from the heat. Tilt it and spoon away excess grease and liquid, then stir the mayonnaise into the beef.
Keep the rolls connected and slice each package horizontally. Place the bottom slabs on the pan and cover them with the sliced cheddar. Spread the beef evenly over the cheese, add the shredded cheddar, and replace the roll tops.
Brush the tops with melted butter and immediately add the sesame seeds. Bake uncovered for 12–15 minutes, until the cheese is melted, the centers are hot, and the roll tops are golden. Divide into 24 sliders and serve hot.