Place cauliflower, milk, onion powder, garlic salt, and pepper in a small saucepan. Cover and bring to a boil over medium heat. Reduce heat to low and simmer for 7-8 minutes, until cauliflower is softened. Remove garlic, mash into a paste and set aside
Meanwhile, add oil, mushrooms onion, garlic paste and whole sprig of thyme to a medium-sized saucepan. Heat over high heat until onions are translucent and beginning to brown, about 8 minutes. Once vegetables have been browned, deglaze the pan by pouring the white wine into the pan and stir until the sauce has reduced and thickened, about 5 minutes. Remove thyme sprig (it has completed its job!)
Pour cauliflower mixture and sauteed mushrooms into a food processor, immersion blender or blender leaving a small handful of mushrooms behind for garnish if desired. Puree until mixture is smooth.
Bring to a boil, cover and simmer for 10 minutes, until thickened.