Preheat your oven to 375°F. While it heats, prepare the rhubarb filling by combining the sliced rhubarb, sugar, flour, cinnamon, and nutmeg in a 9x13 inch baking dish (or similar size). Toss everything together until the rhubarb is evenly coated—the flour will help thicken the juices as the fruit bakes. I like to let this sit for a minute so the sugar starts to dissolve into the rhubarb.

In a separate bowl, combine the butter (cut into small cubes), oats, flour, brown sugar, salt, and cinnamon. Using a fork or your fingertips, work the cold butter into the dry ingredients until the mixture resembles coarse breadcrumbs with some pea-sized pieces of butter still visible. Don't overmix—these little butter pockets create the crispy texture we're after. I find using cold butter straight from the fridge makes this step much easier and gives a better crumble.

Evenly distribute the oat topping mixture from Step 2 over the rhubarb filling, breaking up any large clumps as you go but keeping some texture. Don't pack it down—let it stay loose so air can circulate and create those crispy bits. Bake in your preheated 375°F oven for 30-35 minutes, until the topping is golden brown and the rhubarb filling is bubbling at the edges. The crisp is done when you can see the juices coming up around the sides of the topping.

Remove the crisp from the oven and let it cool for at least 10-15 minutes before serving. This allows the filling to set slightly so it won't be runny, and it gives the topping time to crisp up further as it cools. Serve warm with vanilla ice cream or whipped cream if desired.