Heat the oil in a large skillet over medium heat. Add the onion and celery, season with salt and pepper, and cook, stirring occasionally, until just tender, about 5 minutes. Stir in the rices and 1/4 cup water until the water evaporates and the rices are heated through, about 3 minutes.
Transfer to a large bowl and stir in the lemon zest and juice and mustard. Let cool slightly, then stir in the trail mix and season to taste with salt and pepper.