Place cauliflower, vegetable broth, and herbes de Provence in a large pot, cover, and set to medium heat, for about 12-15 minutes, or until the cauliflower is soft. Remove from heat, let cool slightly before transferring to a blender. Blend for about 45 seconds*, or until velvety smooth. Return to pot.
Meanwhile, heat a separate pan over medium-low, add oil, and sauté onions for 2 minutes, before adding chopped mushrooms. Sauté another 7-10 minutes, until mushrooms are well cooked.
Add the mushrooms to the pot with the cauliflower base, and simmer over low for 10 minutes.
*When blending hot liquids, remove the clear center piece in the lid of the blender, and place a dishtowel over the opening. This will allow the steam out. Never fill the jar more than half way, and keep it at a low to medium speed.