Preheat your oven to 350°F (175°C).
Peel, core, and slice the apples. I like a mix of tart and sweet apples for balance.
In a large bowl, toss the apples with both zests, both juices, 1/2 cup sugar, cinnamon, and nutmeg.
Spread the apple mixture evenly in a 9x13-inch baking dish.
In another bowl, mix the flour, both sugars, salt, and oats.
Add the cold diced butter and use your fingers (or a pastry cutter) to mix until crumbly.
Sprinkle the topping evenly over the apples. Don’t press it down—let it fall like apple crisp snow.
Bake for about 60 minutes until the topping is golden and the apple juices are bubbling.
Let it cool slightly (if you can wait), and serve warm with ice cream or whipped cream.