Lightly coat a 9" x 5" loaf pan with cooking spray
In a saucepan over medium-high heat bring chicken stock, onions, and oats to a boil. Reduce heat and allow it to simmer 5 minutes.
Remove stock mixture from heat and stir in thyme, sage, salt, & pepper
Layer sweet potato slices, then apple slices, then turkey slices, in prepared pan.
Pour chicken stock, oat mixture evenly over layered sweet potato, apples, and turkey
In a small bowl, stir together topping ingredients
Spread topping evenly over top of the casserole
Bake, covered with foil, 30 minutes. Remove foil and bake a further 10-15 minutes, until sweet potatoes and apples are fork tender and topping is golden.
Remove from oven and allow to stand 5 minutes before cutting into slices and serving.