How to Make It
Preheat oven to 425°F. Roll 1 of the piecrusts into a 12 1/2-inch circle. Loosely press into a 9-inch deep dish pie plate; crimp edges. Poke surface of piecrust all over with a fork. Freeze until firm, about 15 minutes; transfer to refrigerator until ready to use. Cut remaining piecrust into 10 (3/4-inch-wide) strips. Place strips in refrigerator until ready to use.
Meanwhile, heat a large skillet over medium-high. Add beef in a single layer; cook, undisturbed, until lightly browned, about 2 minutes. Stir beef to crumble; cook, stirring occasionally, until browned and no longer pink, about 5 minutes. Add onion and garlic; cook, stirring occasionally, until softened, about 4 minutes. Reduce heat to medium-low; stir in mayonnaise, Worcestershire, relish, mustard, salt, and pepper until combined. Sprinkle with flour; stir to coat. Increase heat to medium. Add water; cook, stirring constantly, until mixture is thickened slightly, about 1 minute. Remove from heat; stir in lettuce. Let cool about 10 minutes.
Remove piecrust in pie plate from freezer/refrigerator; line with aluminum foil. Fill foil with pie weights or dried beans. Bake in preheated oven until lightly browned, about 10 minutes. Remove foil and pie weights. Return piecrust to oven; bake until bottom is dry, about 5 minutes. Remove from oven; reduce oven temperature to 400°F. Let piecrust cool about 10 minutes.
Spoon one-third of the beef filling into cooled piecrust. Top evenly with 3 of the cheese slices, tearing slices as needed to cover. Repeat process once with one-third of the filling and remaining 3 cheese slices. Top evenly with remaining one-third filling.
Remove piecrust strips from refrigerator. Arrange 5 of the strips vertically over filling. Weave the remaining 5 strips perpendicular into the vertical strips to form a lattice pattern. Brush lattice piecrust with cream; sprinkle with sesame seeds. Place a foil ring around the edge of the pie to prevent overbrowning.
Bake at 400°F until lattice is nicely browned, about 45 minutes. Let cool about 15 minutes. Slice into 6 wedges. Top evenly with ketchup, if desired.