Mix rosemary, thyme and garlic powder in small bowl. Reserve 1/2 teaspoon. Heat 1 teaspoon of the butter in large saucepan on medium heat. Add mushrooms, carrots and remaining seasoning mixture; cook and stir 3 minutes. Remove from saucepan. Set aside. Melt remaining 3 teaspoons butter in saucepan on medium heat, stirring to release browned bits from bottom of skillet. Sprinkle with flour; cook and stir 3 to 4 minutes or until flour is lightly browned.
Gradually stir in broth until well blended. Bring to boil. Reduce heat to low; simmer 10 minutes or until slightly thickened, stirring occasionally. Add vegetable mixture, chicken, pearl onions and peas; simmer 8 minutes or until chicken is cooked through, stirring occasionally.
Meanwhile, preheat oven to 350°F. Cut bread into rounds with 3-inch cookie cutter. Place on baking sheet. Brush bread with oil and sprinkle with reserved seasoning mixture. Bake 10 minutes or until toasted. To serve soup, ladle into soup bowls and top each with 1 crouton.