To make the topping, combine the flour, oats, brown sugar, and cinnamon; stir to combine.
1 cup all-purpose flour, 1 ½ cups old-fashioned oats, 1 cup light brown sugar, ½ teaspoon ground cinnamon
Add the butter, and cut in until the mixture looks crumbly.
½ cup cold salted butter
Crumble the topping over the filling.
Bake for about 45-50 minutes, or until the filling is hot and bubbly all the way through and the topping looks crispy and light golden brown. If the topping begins to brown too quickly, tent with aluminum foil.