Preheat oven to 400 degrees
Salt and pepper chicken breasts and bake 20-25 minutes until cooked through; pull into pieces.
In a large skillet, add half stick (4 tablespoons) butter and minced vegetables and saute on medium high heat until softened.
Add flour and whisk until it forms a paste like consistency and all lumps are gone.
Pour in stock and thyme and whisk until smooth. Simmer for 10 minutes, whisking occasionally.
Remove skillet from heat and stir in shredded cheese, sour cream and poppy seeds.
Stir occasionally and let cheeses melt while you work on the crumb topping.
Crush crackers into crumbs by placing the crackers in a large zip lock bag and smashing with a rolling pin.
In a small bowl, add crushed crackers, and butter; microwave 30-60 seconds until butter is melted and stir to evenly coat the crumbs.
Pour skillet into a 13x9 inch casserole dish.
Sprinkle buttery cracker crumbs evenly over the top.
Bake in a 400 degree oven for 20-30 minutes, until golden brown and bubbly.
Serve with steamed rice.