Preheat oven to 350°F (175°C). Line a 9x13-inch pan with parchment paper.
Combine melted butter, sugars, flour, cinnamon, and salt into a crumbly dough.
Press 2/3 of the dough into the pan and bake for 15 minutes.
Mix diced apples with flour, sugar, cinnamon, nutmeg, and lemon juice.
Spread apple filling over baked crust, crumble remaining dough on top.
Bake another 30–35 minutes until golden and bubbly.
Cool completely and chill before slicing. Enjoy!