Heat the oven to 350°F. Cook beef and onion in a large skillet over medium-high heat for 8 to 10 minutes, breaking the meat into small crumbles, until the beef reaches 160°F. Drain excess fat.
Stir in salt, pepper, ketchup, mustard and drained relish. Split the connected Hawaiian rolls horizontally and place the bottom slab in a parchment-lined 9-by-13-inch pan.
Layer half the cheese over the bread, spread beef evenly and cover with remaining cheese and the top slab.
Brush with melted butter, sprinkle with sesame seeds and cover loosely with foil. Bake for 15 minutes, then uncover for 8 to 10 minutes until the center reaches 165°F.
Rest for 5 minutes, then cut into 12 sliders so the filling has time to settle.