Combine flour, 1/4 cup white sugar, and baking powder together in a mug. Add egg, vegetable oil, water, and 1 tablespoon lemon juice; stir until well mixed.
Cook in the microwave on high until cake is cooked through, 2 1/2 to 3 minutes. Remove from microwave and cool.
Mix 3 tablespoons white sugar and 1 tablespoon lemon juice together in a microwave-safe bowl until smooth; cook in microwave until sauce is bubbling, about 30 seconds. Pour sauce over cake.
