Chicken and Dumpling Casserole

1/2 cup chopped onion
1/2 cup chopped celery
2 garlic cloves, minced
1/4 cup butter or margarine
1/2 cup all-purpose flour
2 teaspoons granulated sugar
1 teaspoon salt
1 teaspoon dried basil
1/2 teaspoon pepper
4 cups chicken broth
1 (10 ounce) package frozen green peas
4 cups cubed cooked chicken

Dumplings
2 cups buttermilk biscuit mix
2 teaspoons dried basil
2/3 cup milk

In large pan, saut