Chicken and Dumplin' Casserole

1/2 cup onion, chopped
1/2 cup celery, chopped
2 garlic cloves, minced
1/4 cup butter
1/2 cup 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 chicken, cooked, cubed

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

In a large saucepan, saut