Brown the meat in a large skillet over medium heat, 8 to 10 minutes. Remove it from the heat and drain off the fat. Add the ketchup, barbecue sauce and mustards to the meat and stir to combine. Taste and add more of whatever you think it needs. Keep warm.
Melt 1/4 stick (2 tablespoons) of the butter in a separate skillet over low heat, then add the bun cubes and toss to coat. Toast the croutons in the skillet over low heat for 10 to 12 minutes, adding 1/4 stick (2 tablespoons) of butter every couple of minutes until you've used the whole stick. (This will ensure all the croutons soak up the butter and toast nicely.)
To assemble: Pile some torn lettuce on each plate. Pile on a good quantity of the meat mixture, then top with plenty of grated Cheddar. Add diced tomato, chunks of pickles and slices of red onion over the top. Finish by sprinkling warm croutons all around the plate. Serve immediately.