Toss the apple slices with lemon juice to prevent browning. Set aside.
In a mixing bowl, whisk together the flour, sugar, baking powder, and salt. Add the milk, egg, and vanilla extract, and whisk until smooth.
In a large skillet or deep fryer, heat oil over medium heat to 350°F (175°C).
Dip each apple slice into the batter, making sure it's well coated. Carefully place the coated apple slices into the hot oil and fry until golden brown, about 2-3 minutes per side. Remove from the oil and drain on paper towels.
Enjoy your meal!