Combine flour and Old Bay seasoning in a shallow dish. Dredge each crab in flour mixture.
Melt butter in a large skillet over medium-high heat until butter begins to brown. Add crabs; cook for 3 minutes, gently pressing body and legs against pan. Turn crabs; cook 3 minutes or until brown and cooked through. Drain on paper towels.
Spread 1 tablespoon Lemon Tartar Sauce over each of 4 bread slices. Top each with 1 crab, 3 tomato slices, and 1 lettuce leaf. Top each with 1 bread slice.