In half inch of water, cook greens till wilted, about 1 minutes. When cool enough to handle, squeeze excess water out. Chop coarsely and let cool more if still warm.
In a medium bowl, mix together mayonnaise, sour cream, dry soup mix, water chestnuts and chopped garlic mustard. Chill in the refrigerator 6 hours, or overnight.
Remove top and interior of sourdough bread. Fill with mixture. Tear removed bread chunks into pieces for dipping.