Cut the cauliflower into florets and cook them for approximately twelve minutes until tender in lightly salted water. Cook the green beans in a separate pan for five to six minutes until tender-crisp.
Meanwhile, mix the ground beef with the egg, finely minced onion and garlic, salt and pepper, and roll them into firm meatballs. Heat the olive oil in a skillet and brown the meatballs on all sides until cooked through, approximately ten minutes.
Drain the cauliflower and mash it with the butter and a pinch of nutmeg until creamy. Serve the puree with the meatballs and green beans on the side.