Toss pears and lemon juice together in a bowl until coated; spread into a 9x9-inch baking dish. Pour maple syrup and 2 tablespoons melted butter over pear mixture; toss to coat.
Mix oats, brown sugar, 1/2 cup melted butter, flour, and cinnamon together in a bowl until crumbly; sprinkle over pear mixture.
Bake in the preheated oven until crust is golden and pears are tender, about 45 minutes.