**Preheat the Oven:**

Preheat your oven to 350°F (175°C). Line a baking sheet or casserole dish with aluminum foil for easy cleanup.

**Prepare the Burger Patties:**

In a large mixing bowl, combine the ground beef, onion, garlic powder, salt, and black pepper. Mix gently until just combined. Form the mixture into 8 equal patties.

**Cook the Patties:**

Heat a skillet over medium-high heat. Sear the burger patties for 2-3 minutes on each side until browned but not fully cooked through. Remove from the skillet and set aside.

**Make the Sauce:**

In a small bowl, whisk together the mayonnaise, ketchup, and mustard until smooth.

**Assemble the Burgers:**

Place the bottom halves of the slider buns on the prepared baking sheet. Spread a small amount of the sauce on each bun, then top with a burger patty and a slice of cheese. Add the top halves of the buns.

**Brush with Butter:**

Brush the tops of the buns with melted butter. Cover the baking sheet loosely with aluminum foil to prevent the buns from over-browning.

**Bake:**

Bake in the preheated oven for 15-20 minutes, or until the cheese is melted, and the patties are cooked through.

**Serve:**

Remove from the oven and let the burgers rest for a couple of minutes before serving.