In Japan, they love raw vegetables with dipping sauces, much like we love crudité platters, chips and salsa, and bagna cauda. Wasabi stems will end up on such platters. If the stem is too tough for your liking, it can be lightly blanched in boiling water to soften it a little. Here is a nice miso dipping sauce:
Walnuts 4 tbsp
White Miso 1 tbsp
Dark Miso 1 tsp
Dashi (or water) 3 tbsp
Puree all the ingredients together in a food processor until smooth. You can adjust the thickness of the dipping sauce to meet your needs.