Combine lemon juice and the water in a small bowl; immerse sliced pears until ready to use to prevent discoloration. Blot dry with paper towels before using.
In a medium bowl, gently stir Roquefort into butter, leaving small bits of cheese. Be careful not to overmix, or the butter will turn blue. Add pepper.
Spread a thin layer of Roquefort butter on two slices of bread. Line one with pears, overlapping slightly, and top with the other bread slice. Use a serrated knife to trim crusts and cut into three rectangles (about 1 by 3 inches). Repeat with remaining ingredients. Cover with damp paper towels until ready to serve.