Pre-heat oven to 375ºF.
In a medium bowl, toss the chicken with the pepper, salt, flour, cornstarch, garlic powder, and thyme.
In a large skillet, over medium-high heat, heat the olive oil.
Brown both sides of the chicken pieces, about 3 to 4 minutes per side; then remove to a plate.
Add the onion and white wine to the skillet and stir to deglaze.
Cover and cook for about 4 minutes, until onions are translucent.
Remove lid and continue to cook at medium-high heat until no liquid remains.
Add the butter and reduce heat to medium.
Continue to cook the onions, stirring frequently with a wooden spoon, until they look golden – about 10 minutes more.
Stir the flour into the now-caramelized onions and cook 2 minutes more; then slowly stir in the broth.
Let the mixture simmer for 2 minutes before turning off the heat.
Place half the toasts in the bottom of a 9 x 1-inch baking dish.
Pour in half of the onion mixture and top with the chicken.
Add half the remaining cheese in an even layer; then add the rest of the onions and broth, and top with the rest of the cheese.
Top with the other half of the toasts, and cover the dish with foil.
Bake for 20 minutes, then remove the foil and bake 5 minutes more.
Garnish with fresh parsley and serve.
