Melt butter in a 3-quart pan over medium heat. Add onion and cook for 2 minutes or until soft. Add fresh shiitake, button mushrooms, oyster mushrooms and minced ginger. Cook for 3 minutes or until mushrooms are soft and pan juices have evaporated. Stir in the cornstarch and cook for 1 minute. Stir in the vegetable broth and half-and-half. Bring to a boil, stirring constantly. Add vegetable oyster-flavored sauce and stir to mix well. Add chopped cilantro and chopped green onions. Place soup in a blender or food processor and process until smooth. Add sesame oil and pepper. Ladle soup into bowls. Garnish each serving with enoki mushrooms, green onion slivers and cilantro leaves.