Start heating your oven to 425 degrees. Peel and chop the apples, removing the core. Chop the rhubarb into slices 1 inch thick.

Combine the apple, rhubarb, sugar, vanilla, and lemon in a bowl. Then stir in the cinnamon, salt, and three tablespoons Flour. Pour the mixture into the raw pie crust.

Then in a clean bowl, combine the remaining flour, brown sugar, and butter with a pinch of salt. Use two forks to blend the mixture into small crumbs. Then pour over the pie.

Bake the pie at 425 degrees for 15 minutes, then reduce the heat to 375 degrees and cook for an additional 30 minutes. Reheat at 225 degrees if preparing the pie ahead of time.

Serve warm with a scoop of vanilla ice cream.