Preheat oven to 375 degrees. Butter the bottom and sides of an 8×8″ baking dish, set aside.
In small bowl, combine flour, oatmeal, brown sugar, baking powder, cinnamon, and salt, stir to combine. Add diced butter. Using a fork or pastry cutter, cut in the butter until it’s pea-size and crumbly. Store in refrigerator until ready to use.
After dicing apples, pour lemon juice over them to prevent browning. Stir to coat apples.
Pour melted butter into a small bowl. Add flour and mix well. Mix in water and vanilla extract. Add brown sugar, cinnamon and salt, stirring until smooth.
Pour mixture over apple slices and stir to evenly coat. Pour into prepared baking dish and spread evenly. Sprinkle crumb topping over apples and bake for 35 minutes until top is golden and inside is bubbly and when poked with a fork, tender.
Remove from oven and let set for at least 10 minutes before serving. Serve with ice cream.