Preheat oven to 350°.
Place a sheet of parchment paper on a cookie sheet.
Finely dice the apples and toss in the lemon juice.
Put the butter and sugars into a bowl of an electric mixer and beat together until creamy.
Gradually add in the beaten egg.
Stir in the flour, baking powder, the oats, and the apples. Mix until well combined.
Place the dough by rounded Tablespoons on the cookie sheet.
Bake for 12-15 minutes, or until golden around the edges.
Let cool on the baking sheet for 3 minutes, then place the cookies on a wire rack to cool completely.