Add butter to your saucepan on medium heat. Once sizzling, add onions and stir until well coated. Season with a pinch of kosher salt.
Lower the heat to low and stir every few minutes for 20 minutes. Cook for another 10 minutes, stirring more often until dark brown.
Preheat oven to 350.
Shape ground beef into a rectangle that is slightly larger (about 1 inch all around) than your 6 slider buns. Heat a large pan (or griddle) on high. Transfer beef to the pan/griddle and cook for about 5 minutes per side.
Remove and let rest for 5-10 minutes.
Place the cooked beef on the bottom of the bread. Top with gruyere, Dean’s French Onion Dip, and caramelized onions. Place top of bread back on top and bake for 10-15 minutes or until cheese is melted.