Heat the oven to 350°F and grease the bottom of a rimmed baking sheet with butter. The pan should have a light, even coating so the bottom buns release easily after baking.

Warm the olive oil in a large skillet over medium-high heat, add the diced onion and ground beef, then season with salt, black pepper, and garlic powder while breaking up the meat with a spatula. Cook until the beef is just done and no pink remains, and look for the onions to smell sweet and the skillet to sound less wet and more sizzly.

Tilt the skillet and spoon off the excess fat, then stir the mayonnaise into the hot beef mixture. This is the key step for avoiding soggy sliders, and the finished meat should look moist and cohesive rather than greasy.

Slice the dinner rolls in half and set the bottom half on the prepared baking sheet, then cover them with the sliced cheddar, spread the beef evenly over the cheese, and finish with the shredded cheddar. The filling should reach the edges in an even layer so every slider gets the same amount of meat and that gooey top cheese melt.

Set the top buns cut-side down over the filling, brush the tops with melted butter, and sprinkle on the sesame seeds right away before baking for 12 to 15 minutes at 350°F. The sliders are ready when the tops are golden brown and the cheese underneath looks fully melted and lava-like.