Heat a grill pan or skillet over medium heat. Brush the outside of the hot dog buns with the butter. Toast the buns until golden brown, about 1 minute per side, and set aside.
Increase the heat to medium-high. Halve the hot dogs lengthwise, making sure to not slice all the way through. Place the hot dogs cut-side down and grill until browned and nicely charred, about 4 minutes. Flip and cook until charred, another 4 minutes.
Meanwhile, heat the baked beans in a small saucepan over medium-low heat until just heated through, about 5 minutes.
Heat the crispy onions in a small saute pan over low heat until warmed through and extra crispy. Season with a pinch of salt.
Place the hot dogs in the toasted buns and layer each with about 3 tablespoons baked beans, some crispy onions, banana peppers and a drizzle of mustard. Serve hot.
