To start making vegetable ceviche, place the chopped celery, onion, garlic and cilantro into a bowl. Juice the limes and add to the top of the veggies along with the salt and sugar.
Use a pestle to crush the vegetables and release the juice in the veggies. Add the torn nori sheet to the bowl and mix it up. Place into the refrigerator for 30 minutes.
Slice the mushrooms, zucchini and artichokes into very thin slices. Slice the red onions into fine julienne and set aside separately from the other vegetables. Dice up the chile pepper, removing the seeds and veins for less heat and set aside.
Just before serving, add the marinade to the bowl of mushrooms and veggies and mix together well. Add the red onions and mayonnaise. Mix it all up. Taste and adjust seasonings Divide up the ceviche into four bowls and garnish with sweet potatoes and white corn.