In a large stock pot, bring water to a boil and add in chicken breasts. Lower to a simmer and cook for 25-30 minutes.
In a small saucepan, heat 2 T. butter and sautee mushrooms.
Add chicken broth on low heat. Stir in milk.
Slowly whisk in flour. Once the sauce begins to thicken, remove from heat.
Stir in Greek yogurt and season accordingly, if necessary, with the onion and garlic powder, pepper, and 1/4 t. Salt. Set aside.
When chicken is cooked, shred chicken and place into a bowl.
Combine chicken with set aside sauce and an additional 1/4 t. salt. Then fill an 8x8 glass pan.
Melt remaining butter in a small dish. Stir in panko and poppy seeds well until they form small clumps.
Spread the topping over the chicken.
Bake at 375 degrees Fahrenheit for 30 minutes, or until topping has browned and sauce is bubbly.