Combine water and chicken and bring to a boil in a stock pot or Dutch oven. Add carrots and bring to a second boil. Reduce heat and simmer until crisp tender, about 7 to 8 minutes.

Add tortellini, mushrooms, chicken, onion, salt, tarragon, garlic and pepper; simmer 10 to 12 minutes. Do not let soup boil. Ladle into bowls. Sprinkle Parmesan cheese over each serving.

Slice bread lengthwise in half. Spread butter and mustard on cut surface. Sprinkle with garlic and pepper. Combine cheese and green onions in small bowl; sprinkle on bread.

Place on broiler pan. Broil 3 inches from heat for 3 to 5 minutes, or until cheese is melted and bubbly. Slice to serve.