White Sauce Chicken Enchiladas

1 lg. onion, chopped
1 tsp. ground coriander
2 tbsp butter
8oz sour cream
2 c. cheddar cheese, grated
1 c. jack cheese, grated
2 cans sliced olives
1 tsp garlic salt
1/4 tsp pepper
3 Tbsp flour
2 c. chicken broth
2 lg. tomatoes, chopped
4-5 chicken breasts, cooked and shredded
1 can black beans, drained & rinsed (opt)
12 flour tortillas

Sauce:
(My mom always doubles the sauce)
Saute butter, onion, garlic, salt, pepper and coriander until onion is tender.  Stir flour into sour cream and add to onion mix.  Add chicken broth and cook until bubbly. Add 1 cup of cheddar cheese and 1/2 cup Jack cheese; stir to melt.

In separate bowl, add chicken, 1 can olives, and 1/2 tomato (keep some olives and tomatoes to top enchiladas).  Add 1 c. onion sauce mix to chicken.  Add in black beans. Fill tortillas with chicken mix.  Spray pan.  Place enchiladas in 9x13 pan and pour remaining sauce on top.  Cover and bake 35mins at 375*F.  Uncover; add remaining tomatoes, olives, and cheese.  Bake 5 more mins.  Serve.