Preheat the oven to 350 degrees. Butter a 9" x 13" baking dish.

To make the filling, cut the nectarines in half, and then cut them into 1/4 inch thick slices. Cut the stems off the berries and cut the large ones in half, leave the small ones whole. Put the fruit in a large bowl.

In a small bowl, combine the sugar and flour. Add the butter and lemon juice to the fruit in the large bowl and then sprinkle the sugar/flour mixture over it. Gently combine. It may look and feel a little "pasty," but as the fruit cooks it's going to release a lot of liquid which will thicken with the flour and butter.

For the crisp topping, cut the cold butter into small pieces, about 1/2". In a medium sized bowl, combine the brown sugar, flour, rolled oats and cinnamon. Add the butter and mix it together with your hands or a pastry cutter until the texture resembles a coarse meal. There should still be tiny pieces of butter so don't mash it all into one lump. Spread this over the filling.