In a large bowl, whisk together the sugar, corn starch, and ground nutmeg to combine evenly and prevent lumps. Add the cut rhubarb and sliced apples to the bowl, then gently toss everything together until the fruit is evenly coated with the sugar mixture. Let this sit while you prepare the topping—the fruit will begin releasing its juices and the corn starch will help thicken them during baking. I like to prep my fruit this way because it ensures the filling won't be watery.

In a separate bowl, combine the oats, brown sugar, cinnamon, and flour with a fork or your fingers. Working quickly, cut the cold butter cubes into the dry mixture using a pastry cutter, two knives, or your fingertips, breaking the butter into pea-sized pieces. The mixture should look crumbly and cohesive when squeezed together. Stop mixing once the butter is evenly distributed—overworking it will make the topping dense instead of crispy.

Preheat your oven to 350°F. Pour the fruit filling from Step 1 into a greased 9x13 inch baking dish (or similar size), spreading it into an even layer. Evenly distribute the crisp topping from Step 2 over the fruit, pressing it down gently so it holds together but doesn't compact too much. Bake for 50 minutes, until the fruit is tender and the topping is golden brown. For a crispy texture, I avoid stirring the topping during baking—let it bake undisturbed so it develops a nice crust.

Remove the crisp from the oven and let it cool for 5-10 minutes at room temperature. This allows the filling to set slightly so it won't be too runny when served, while the topping will still be warm and crispy. Serve warm, optionally with vanilla ice cream or whipped cream.