chicken and biscuits casserole

INGREDIENTS

large chicken breasts, about 1 pound 4 oz.

3 tbsp

canola oil, divided 

2 cups

diced onion

1 1/2 cups

diced carrots

1 1/2 cups

chopped celery 

3 cloves

garlic, minced 

1 tsp

salt, divided 

1/2 tsp

freshly ground pepper

1 1/4 cups

all-purpose flour, divided

3 cups

reduced-sodium chicken broth 

1 cup

frozen peas 

2 tbsp

chopped mixed fresh herbs, such as sage, parsley, thyme and/or chives, divided

1 cup

white whole-wheat flour, or whole-wheat flour 

1 tsp

baking powder 

1/2 tsp

baking soda 

egg 

1 cup

buttermilk