In a large fry pan, melt butter on medium heat adding the onion and garlic. Cook until soft and translucent for approximately 5 minutes and then add ketchup, chicken stock, Worcestershire and thyme. Continue to simmer for about 10 minutes.
To make the patties, place the ground turkey in a large bowl and combine with the egg, panko bread crumbs and half of the cooked onion mixture. Form 8-9 patties and using a melon baller, scoop out an indent in the center of each slider, fill with the shredded cheese and replace the scooped out ground meat mixture over the cheese so that the cheese is now in the middle of the patty. Grill the sliders approximately 5 minutes over medium heat on the one side, flip and grill on the other side for an additional 5 minutes or until done all the way through. Place the cooked patties on slider sized buns and top with the remaining onion mixture.
Tip: If you would like to make this as a stuffed meatloaf, pat out the prepared meat mixture on a sheet of waxed paper in the shape of a rectangle. Spread the grated cheese over the meat mixture and using the waxed paper to lift the one end of the rectangle, begin jelly rolling the meat mixture using the waxed paper to assist (but don’t roll the paper up in the meat!) Pinch the ends closed and place in a loaf pan for baking in an oven at 350 degrees for approximately 1 hour or until done in the center. Garnish with the remaining cooked onion.