Preheat the oven to 375°F (190°C).
Heat the olive oil in a large skillet over medium heat.
Add the diced onion and cook for 3-4 minutes until translucent.
Add the minced garlic and cook for 1 minute until fragrant.
Add the chicken pieces to the skillet and cook for 5-7 minutes until no longer pink on the outside.
Add the broccoli florets, sliced mushrooms, and diced red bell pepper to the skillet and cook for 4-5 minutes until vegetables are slightly tender.
Stir in the dried thyme, dried oregano, black pepper, and salt.
Pour in the chicken broth and bring to a simmer, cooking for 3 minutes.
Remove the skillet from heat and stir in the Greek yogurt and shredded cheddar cheese until well combined.
Transfer the mixture to a 9×13 inch casserole dish and spread evenly.
Sprinkle the grated Parmesan cheese and whole wheat breadcrumbs evenly over the top.
Bake in the preheated oven for 25 minutes, or until the top is golden brown and the casserole is bubbling.
Remove from oven and let cool for 5 minutes before serving.