Heat the oven to 350. Spray a 2 quart casserole with cooking spray.
Bring a medium saucepan of water to boil. Add cauliflower and cook until tender, about 10 minutes. Drain and transfer to a large bowl.
Meanwhile, in a large skillet, heat oil over medium heat. Add onion and garlic; cook, stirring occasionally, until translucent, about 5 minutes. Add beef and brown for 10 minutes, stirring to break up lumps. Add edamame and cook, stirring occasionally, 3 minutes longer. Stir in broth and Worcestershire sauce. Season with pepper and a pinch of salt. Transfer meat mixture to the casserole.
With an electric mixer at medium speed, whip the cooked cauliflower with sour cream, egg yolk, and another pinch of salt. Spoon cauliflower evenly over meat. Top with cheese and bake for 20 to 25 minutes, or until golden on top. Serve warm.