In a large casserole, melt butter at medium-low heat. Add leeks and cook for 4 to 6 minutes.
Slice mushrooms base and throw it away. Wipe mushrooms heads with moisted paper towels and slice finely. Add mushrooms to leeks. Cook while stirring, until mushrooms melt a bit, so about 2 minutes. Add broth and bring to a boil at high heat. Add garlic, ginger, teriyaki sauce and sherry. Reduce at medium low heat. Cover and simmer while stirring few times for about 10 minutes.
Remove ginger slices from the soup. Add sesame oil. Add finely chopped chive. Divide soup into bowls and spread with sliced leeks.
Chef’s note
Covered and refrigerated, this soup can be refrigerated until 3 days and frozen 3 months.