- Preheat the oven to 350 °F (175 °C). 
- In a 12×8 (or similar) baking dish, place the peeled and diced apples. Toss together the 3 Tbsp flour, granulated sugar, cinnamon, and nutmeg with the apples until evenly coated. 
- In a medium bowl, mix together flour, oats, brown sugar, baking powder, salt, and your crushed walnuts and pistachios. 
- Cut in the cold, cubed butter using a fork, pastry cutter, or your fingers until the mixture resembles coarse crumbs or “pebble-like” bits. 
- Sprinkle (or gently pat) this nut-enhanced streusel topping evenly over the apple layer. 
- Bake for 30–35 minutes, or until the top is golden brown and the apples are bubbling. 
- Serve warm with optional sea salt flakes, vanilla ice cream, or a drizzle of salted caramel.