Spread mincemeat evenly over bottom of unbaked 10-inch pastry in pan. Set pan in a foil-lined 12-inch pizza pan or 10- by 15-inch baking pan.
Grate 1 tablespoon of peel (yellow part only) from the lemon. Ream 3 tablespoons juice.
In a bowl, mix sugar and flour. Whisk in milk, butter, eggs, lemon peel, and lemon juice. Carefully pour mixture over mincemeat.
Bake in a 375° oven until custard barely jiggles when pan is gently shaken, 45 to 55 minutes (35 to 45 minutes in a convection oven). If pie browns too quickly (check after 15 minutes), cover loosely with foil. Let cool on a rack, uncovered, about 2 hours. Cut into wedges.