Preheat the oven to 350F/180C and spray an 13 x 9 inch pan with cooking spray.
Place apples, pears, cornstarch, lemon juice, brown sugar and cinnamon into a medium sized bowl and toss until all the fruit is coated in the cornstarch/sugar mixture. Leave to one side.
Place the oats, sugar, flour and salt into a medium sized bowl and stir briefly to combine.
Add in the cubes of butter and vanilla, then rub between your fingers until you have a coarse breadcrumb texture. Some larger lumps the size of peas is great; adds texture to the crumble!
Tip the fruit mixture into your prepared pan and smooth into a level layer with a spatula/wooden spoon.
Sprinkle the crumble topping all over the fruit into as even a layer as you can get it.
Place in the oven for 35-45 minutes, until the fruit filling is bubbling and the top is golden brown and firm to the touch.
Leave to cool for 20 minutes before serving.
Serve with ice cream and enjoy!