Preheat the oven to 375 degrees and position the oven rack one level below the center. Butter a 12 by 8-inch baking dish (or medium casserole dish with a similar size), and set aside.
For The Filling
Mix the apples with the lemon juice in a small mixing bowl. Add the flour, brown sugar, sugar, cinnamon, nutmeg, and salt and toss to evenly coat. Mix in the melted butter and vanilla.
Pour apple mixture into a prepared baking dish and spread into an even layer.
For The Topping
Mix the flour, brown sugar, cinnamon, and salt in a medium mixing bowl. Add the warm melted butter and stir to blend. Squeeze a handful of the topping into your hand, if it doesn't clump together, add some more melted butter until it does.
Hand-squeeze small handfuls of topping together and crumble clumps evenly over top of apples.
Bake in the preheated oven until the top is golden brown and apples are tender when pierced with a fork, about 30 minutes.
Be sure to cut the apples into similar-sized pieces for even cooking.
Coat the apples in lemon juice as soon as they're cut to stop them from browning.
Test the tenderness of your apples by inserting a fork if it goes in easily, the apples are cooked.
If the fork meets resistance with the apples, but your topping is brown, cover the pan with foil so the apples will bake without your topping burning.