To prepare the onion strings—slice the onion very thin. Place into a baking dish and pour buttermilk over the onions. Let soak for at least an hour.
In a pie dish or large bowl combine flour, salt, cayenne pepper, and ground black pepper. Heat oil in a medium saucepan to 375 degrees F. Take a handful of the onions and toss in the flour mixture. Shake off any excess flour. Place into the hot oil and fry for a few minutes until golden brown. Drain on a plate lined with paper towels. Repeat until onions are gone.
Add bacon slices to a large skillet and cook over medium-high heat, turning once, until brown and crispy. Drain on a plate lined with paper towels. Drain any excess grease from the skillet.
Form the ground chuck into small patties. Season on both sides with salt and pepper. Place the patties into the pan that the bacon was cooked in and cook over medium-high heat, turning at least once, until they are fully cooked (the internal temperature has reached 160 degrees F.). During the last few minutes of cooking top each patty with 2 of the cooked bacon slice halves and 2 of the cheddar cheese squares.
Cut slider buns in half and spread butter on the cut side of each bun. Place cut side down into a clean skillet over medium heat and cook until golden brown on the bottom.
To assemble the sliders—place each slider patty onto the top of the bottom half of each bun. Top with onion strings, a little bbq sauce, and the top half of the bun.