Preheat oven to 225°F.
Cut both roasts into one-inch pieces. Using a food processor, chop meat a handful at a time by pulsing ten to twelve times. This should be enough to grind it perfectly.
Fold the meat together in a large bowl and form 12 burger patties. Brush patties on one side with melted butter and then sprinkle on the blackening seasoning liberally. Repeat on the other side.
Cook to your liking (medium, medium-well), flipping halfway through cooking time. Place the bread in the oven to crisp up for just a minute. Place the burger patties on the bread. Top with lettuce, tomato and mayo.