Melt two tablespoons of the butter in a large skillet over medium heat. Add onions and reduce heat to medium low. Cook for about 30-40 minutes, stirring occasionally until onions are lightly caramelized.
In a large bowl, mix beef together half of the beef bouillon with salt, pepper, and half of the Worcestershire sauce.
Preheat oven to 350℉ and lightly grease a 9x13 baking dish. Add the bottoms of the slider buns to the basking dish.
Add the beef to the skillet with the onions, increasing heat to medium high, breaking apart as it cooks. Cook until beef is browned and drain off any excess grease.
Transfer the beef to the baking dish with the bottom buns and spread evenly over the buns.
Top with gruyere cheese and cover with the top buns.
In a microwave safe bowl, melt butter with the remaining beef bouillon. Stir in the remaining Worcestershire, the thyme, the garlic powder & the minced onions. Sprinkle poppy seeds over the top.
Bake the sliders for 20-30 minutes or until buns are toasted and cheese is fully melted.
