In a small bowl, combine the cut rhubarb pieces with both sugars, vanilla extract, orange zest, cinnamon, flour, and nutmeg. Stir until the rhubarb is evenly coated with the sugar mixture and the flour is fully incorporated. This coating helps the rhubarb release its juices and thicken the filling as it cooks. Transfer the mixture directly into a microwave-safe mug, pressing it down gently to create an even layer.

In a separate small bowl, combine the oats, flour, brown sugar, sugar, and cinnamon. Pour the melted and cooled butter over the dry ingredients and stir until the mixture is crumbly and evenly moistened. I prefer using old-fashioned oats here because they stay chewy and substantial rather than turning into paste during the short microwave time. The butter should coat all the ingredients without making it wet or clumpy.

Spread the crispy topping mixture evenly over the rhubarb filling in the mug, using the back of a spoon to distribute it in an even layer. Microwave on high for 2 to 3 minutes—start checking at 2 minutes to see if the butter has melted and the filling is bubbling around the edges. The mug will be very hot when it comes out, so use caution when removing it from the microwave.

Let the crisp cool for a few minutes before eating—this allows the filling to set slightly and makes it easier to enjoy without burning your mouth. For extra richness and to balance the tartness of the rhubarb, top with a dollop of whipped cream and a light dusting of cinnamon. I find the cream really transforms this into something special and worth the extra step.