In a preheated skillet with 1 tbsp of butter, add in the sliced onion, mushrooms and bell peppers and sauté them until they turn golden brown. Remove them from the skillet.
To the same skillet, add in the sliced beef and brown it until it turns golden brown. Season it to taste with salt and pepper and set it aside.
Place the bread buns onto a baking sheet and top them with beef, followed by vegetables and cheese. Place the top buns on.
Melt 2 tbsp of butter and press the garlic into it. Brush the tops of the buns with the garlic butter mixture.
Bake the Philly cheesesteak sliders at 350°F for 15 minutes or until the outside is golden and the cheese is all melted.
Serve the Philly cheesesteak sliders right away and enjoy!