Heat oven to 400°F. Lay bacon on a rimmed sheet pan and bake for 15 to 20 minutes until crisp, then drain and cool completely.
Toast only the bagel cut faces under the broiler for 1 to 2 minutes and let cool on a rack.
Put cooled bacon, dry chopped lettuce and seeded tomato cubes on a large clean cutting board. Chop together into small even pieces without turning the vegetables into a puree.
Transfer to a bowl and fold in mayonnaise, Dijon and pepper just until coated. Divide the filling evenly among the four bagel bottoms.
Close with the tops and cut in half, holding gently so the chopped filling stays inside. Serve immediately for the crispest lettuce.