Heat butter or olive oil in a 4-quart Dutch oven or other large saucepan over medium heat. Add the onion, a pinch of sea salt, and black pepper. Cook, stirring often, until onions are translucent and very supple, 10 - 12 minutes. Add garlic and cook 1 minute more. Add 2 cups stock or broth, cauliflower, and bay. Bring mixture to a gentle simmer, cover, and cook until cauliflower is tender, about 10 minutes. Uncover pot and cook at a gentle simmer for about 20 minutes longer, until liquid has reduced by about three-quarters. Remove bay leaf.

Meanwhile, sauté shallots and mushrooms. Set a large skillet over medium-high heat. Add 1 tablespoon olive oil and the shallots. Sprinkle with a pinch of sea salt and red pepper flakes. Stirring frequently, cook until shallots are a rich golden brown and crispy, 10 - 12 minutes. Edge heat down if things starts to get smoky. Remove shallots to a paper towel lined bowl.

Add remaining olive oil to the same skillet over medium/medium-high heat. Add half the mushrooms, half the garlic, and half the zest. Sauté until mushrooms release their water and the edges crisp up. Remove to a bowl and repeat for second batch.

In a blender or with an immersion blender, puree the cauliflower mixture until quite smooth and velvety. Spoon back into the Dutch oven and set over low heat. Add sea salt and black pepper to taste. If mixture is too thick, thin with a splash of stock or broth.

To serve cauliflower puree, top with sautéed mushroom, crispy shallots. Top with minced sage and garnish with a few leaves.