In a large bowl, gently mix together the crab meat, bread crumbs, mayonnaise, Dijon mustard, lemon juice, Old Bay seasoning, salt, and pepper until just combined.
Form the mixture into 4 equal patties.
In a skillet over medium heat, melt the butter. Add the crab cakes and cook until golden brown, about 4-5 minutes per side.
Toast the brioche buns in the skillet until lightly browned.
Assemble the sandwiches by placing a lettuce leaf and tomato slice on each bun, followed by the crab cake. Serve immediately.
Notes
For an added kick, serve with a spicy aioli or tartar sauce on the side.