In a bowl toss peeled, cored, and sliced pears with the brown sugar, lemon juice and optional cinnamon. Place in a low sided gratin type casserole dish.
Place the flour, oats, brown sugar, a pinch of salt and the butter in the bowl. Using your fingers, work the butter through the other ingredients until a coarse crumb forms. Sprinkle this over the top of the pears.
Bake for 50-60 minutes or until golden and bubbly.