1. Steam fresh cauliflower until crisp and slightly tender. Don’t overdo it. Alternately, defrost frozen cauliflower. Set aside.
2. Preheat oven to 400 degrees Fahrenheit (200 degrees Celsius). In a small saucepan, sauté onion until golden. Add flour and mix together. Slowly add water and bring to a boil.
3. Lower flame and add salt and pepper (be generous with the seasoning, as cauliflower is bland on its own), mustard, and egg yolk. (Add a little hot liquid to egg yolk first to temper it and then add to pot.) Whisk all ingredients together and cook for a few minutes until thick and creamy.
4. Pour over cauliflower pieces to coat evenly. Place in a large pan and bake uncovered for 15–20 minutes until slightly browned. You might want to baste sauce over vegetables during baking. Serve hot.