Prepare a 9 x 9 inch square baking dish by lightly greasing with butter.

Preheat oven to 350 F and arrange rack in the middle.

Peel and core apples and pears. Cut into either slim wedges or chunks.

Add apples, pears, and lemon juice to a bowl. Toss to lightly coat.

Add the remaining filling ingredients to the bowl and toss to evenly mix. Transfer filling to prepared baking dish.

Using the same bowl, combine the flour, brown sugar, oats, and walnuts. Stir together until evenly mixed.

Add the diced butter. Using either your hands or a fork, mush the butter into the dry mix until a crumbly dough has formed. Sprinkle topping onto fruit.

Bake until fruit is bubbly and fork tender, and topping has browned, about 45 minutes. Let rest 5 minutes before serving.