Position a rack in the center of the oven and preheat to 375º.

If the apple skins are red and pretty, leave them on; or peel them, it's your call. Cut the apples off the core and into 1" chunks. Toss in a very large bowl with the rest of the filling ingredients and scrape into a 9 x 13" casserole, or a gratin dish of equal dimensions.

In another large bowl, combine the oats, flour, sugar, cinnamon and salt (and nuts, if using). Stir to combine, then add the melted butter and work with your fingers until it begins to clump into a gravelly texture. Sprinkle over the fruit.

Bake in the oven for about an hour, until the crisp in golden brown and the fruit is bubbling up furiously around the sides. Remove and let stand for at least 10 minutes. Serve hot, warm, or at room temperature with scoops (or quenelles) of vanilla ice cream.

Store any leftover crisp in the fridge for up to four days; but reheat before serving.