Place cauliflower, milk, onion powder, 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. Then, puree using a food processor, immersion blender or blender.

Meanwhile, add oil, mushrooms and onion to a medium-sized saucepan. Heat over high heat until onions are translucent and beginning to brown, about 8 minutes.

Add pureed cauliflower mixture to sauteed mushrooms. Bring to a boil, cover and simmer for 10 minutes, until thickened.

Serve immediately.