Preheat the Oven:

Preheat your oven to 350°F (175°C).

Cook the Beef:

In a large skillet, heat 1 tablespoon of olive oil over medium heat.
Add the chopped onion and minced garlic, sautéing until softened.
Add the ground beef, cooking until browned. Drain excess fat.
Stir in Worcestershire sauce, Dijon mustard, salt, and pepper. Mix well.

Prepare the Sliders:

Slice the slider buns in half.
Place the bottom halves on a baking sheet.
Evenly distribute the cooked beef mixture over the bottom halves of the buns.

Add Cheese:

Sprinkle cheddar and mozzarella cheese evenly over the beef mixture.
Place the top halves of the buns over the cheese.

Top the Sliders:

In a small bowl, mix the melted butter with remaining olive oil.
Brush this mixture over the top of the slider buns.
Sprinkle sesame seeds, grated Parmesan cheese, and fresh parsley on top.

Bake:

Cover the sliders loosely with foil and bake in the preheated oven for 15 minutes.
Remove the foil and bake for an additional 5-10 minutes, or until the cheese is melted and the tops are golden brown.

Serve:

Remove from the oven, let cool slightly, and serve warm.