Preheat oven to 350F. Spray an 8-x 8-inch baking dish or 11- x 7-inch baking dish coated with nonstick cooking spray; set aside. (I used an 11- x 7-inch dish).
In a large bowl combine apples, sugar, flour, cinnamon, vanilla, and lemon juice. Toss to combine until all apples are covered with flour. Transfer to prepared baking dish.
In a medium bowl combine oats, brown sugar, cinnamon, baking powder, salt, and butter. Use a pastry cutter or two forks to incorporate the butter into the ingredients until it is crumbly. Spread topping evenly over the apples.
Place baking dish on a cookie sheet. Bake in the preheated oven until the top is golden brown and apples are tender when pierced with a toothpick, about 40 to 45 minutes.
Remove from oven and allow to rest 10 minutes before serving. Serve warm with vanilla ice cream, if desired.