Place the beef in a large pot over medium-high heat and cook until browned, 7 to 10 minutes. Drain off the fat. Add the green peppers and onion and cook, stirring, until they soften slightly and are combined with the beef, a couple of minutes. Add the garlic and stir into the mixture for another minute or so.
Add the ketchup and 1 cup water and stir to combine. Add the chili powder, brown sugar, dry mustard, red pepper flakes, Worcestershire and hot sauce. Season with a pinch of salt and pepper. Stir and bring to a simmer. Reduce the heat to medium-low and cook until the sauce has thickened, about 20 minutes.
Spoon a good amount of the meat on the bottoms of the slider buns. Place a good pinch of the fried onions over the top and place the tops of the buns on the sliders. Serve with the red grapes on the side.