Preheat your oven to 400 degrees and spray an 8x8 baking dish with cooking spray.
Using a knife and cutting board, peel the apples and pears if desired. Then, thinly slice the apples and pears.
Juice and zest the lemon.
In a large bowl, mix together the apples, pears, blueberries, lemon juice, lemon zest, half of the brown sugar, corn starch, and salt. This is your crumble filling.
Next, spread the filling into the 8×8 baking dish.
In another bowl, combine the oats, flour, the remaining brown sugar, cinnamon, nutmeg, and melted butter. This topping mixture will be crumbly, and that’s how you want it!
Then, sprinkle the topping on top of the filling. It does not need to cover the whole surface.
Finally, bake for 30-35 minutes until the topping is golden brown.
Serve warm with ice cream, yogurt, or whipped cream. This can also be served cold if desired.