Preheat oven to 350° and grease a 9x13 baking dish. Spread about 1/2 cup of enchilada sauce on the bottom of the baking dish.
In a skillet warm a small amount of oil. Saute onion and garlic until translucent. Add chicken, seasonings and about 1/2 cup green enchilada sauce. Warm through.
Soften tortillas (I use the microwave for about 15-18 seconds), just enough to make them pliable. Mix chicken and 1 cup cheese. Spread a small amount of chicken-cheese mixture on the edge of tortilla. Roll up in tortillas and place in pan over green sauce. Repeat. Spread a thin layer of green enchilada sauce over enchiladas to keep tortillas moist.
In a sauce pan (I usually just reuse the chicken skillet), melt butter, stir in flour and cook 1 minute. Add broth and whisk until smooth. Heat over medium heat until thick and bubbly.
Stir in yogurt and chilies. Do not bring to boil, you don't want curdled yogurt. Pour over enchiladas and top with remaining cheese. Drizzle any remaining green enchilada sauce over cheese.
Bake 30 min and then under high broil for 3 min to brown the cheese. Let set for 5-10 minutes.