Position a rack in the lower third of the oven and preheat to 350ºF.
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-inch chunks; you should have 4 cups. Toss the apples and rhubarb in a very large bowl with the ginger, vanilla, salt, flour, and maple syrup. Scrape into a 9 x 13-inch baking dish, an 8 x 12-inch gratin dish, or the equivalent.
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 the topping over the fruit.
Bake the crisp 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 of vanilla ice cream.
Store leftover crisp in the refrigerator for up to four days. Reheat before serving, or enjoy cold with yogurt for breakfast.