Preheat your oven to 350°F (175°C).
In a large skillet, heat the extra virgin olive oil over medium heat. Add the finely chopped onion and minced garlic, sautéing until soft and fragrant, about 3-4 minutes.
Add the lean ground beef to the skillet. Use your spatula to break it up as it cooks. Stir in the ketchup, yellow mustard, Montreal steak seasoning, and Worcestershire sauce. Cook until the beef is browned and fully cooked through, about 5-7 minutes. Remove from heat and set aside.
While the beef mixture cools slightly, slice the slider buns in half horizontally, keeping them attached. Place the bottom half of the buns in a greased baking dish.
Spread the beef mixture evenly over the bottom half of the slider buns. Place a slice of cheddar cheese on top of the beef mixture for each slider, then cover with the top half of the buns.
In a small bowl, melt the salted butter and brush it over the tops of the slider buns. Sprinkle sesame seeds on top.
Cover the baking dish with aluminum foil and bake in the preheated oven for 15 minutes. Remove the foil and bake for an additional 10 minutes, or until the cheese is melted and the tops are golden brown.
Remove the sliders from the oven and let them cool for a few minutes before slicing them apart. Serve warm with your favorite condiments.