I found this yummy, easy and versatile recipe on the fun food blog:

http://www.hostthetoast.com

I grated the cheese, caramelized the onions, sautéd, drained and seasoned the beef the day before and kept everything in the fridge until I was ready to assemble the sliders and bake.  I LOVE preparing food components ahead of time; it makes entertaining a breeze. The only thing I would change in the recipe below is the butter mixture brushed on the buns- I personally would cut back on the beef base as it made it a tad too salty for me..then again, it did impart a real “french onion” flavor..and anyone who has had french onion soup knows how salty that is!

This recipe is extremely versatile. Next time I make it, I may add thinly sliced mushrooms.  I imagine these sliders could be “Philly-fied” by creating mini Philadelphia cheese steak sliders with chopped beef, onions and Provolone. In the final analysis, what makes this recipe a winner is the concept of lining the pan with mini buns, loading on the fillings, topping with the bun-tops, baking and creating pull apart mini- servings. So easy!

FRENCH ONION BEEF SLIDERS 

INGREDIENTS:

• 8 tablespoons (1 stick) butter, divided
• 2 large sweet onions, sliced
• 2 pounds ground beef
• 2 tablespoons beef base (such as Better than Bouillon), divided
• 1 tablespoon Worcestershire sauce, divided
• Kosher salt and black pepper, to taste
• 24 slider buns
• 12-16 ounces gruyere cheese, grated
• 2 tablespoons sesame seeds (optional)
• 1 tablespoon fresh thyme
• 1 teaspoon onion powder
• 1 teaspoon garlic powder

INSTRUCTIONS

1. Melt 2 tablespoons of the butter in a skillet over medium heat. Add in the onions and reduce the heat to medium low. Cook, stirring occasionally, until lightly caramelized, about 25-30 minutes.
2. Next, brown the beef, drain the fat and add half the Worcestershire sauce. Season with salt and pepper and one tablespoon of beef base.
3. Add the beef to the onions.
4. Preheat the oven to 350°F. Lightly grease a large baking sheet. Separate the tops from the bottoms of the slider buns. Fit the slider bottom buns tightly on the baking sheet.
5. Evenly distribute the beef and onion mixture over the bottom buns. Top with the gruyere cheese. Cover with the top buns.
6. In a microwave safe bowl, melt the butter with the remaining beef base. (I would use 1 tsp instead of 1 tablespoon.) Stir in the remaining Worcestershire sauce and continue to mix until well-combined. Using a pastry brush, brush the beefy butter mixture over the top buns. Sprinkle with the sesame seeds, thyme, onion powder, and garlic powder.
7. Bake the sliders until the cheese is fully melted and the tops of the rolls are lightly browned, about 20 minutes. I served the sliders right in the pan so they stayed warmer longer. You may need a small knife or serving utensil to separate the individual buns.