Preheat oven to 450 degrees. Line a baking sheet with foil and spray it with the cooking spray.
Lay the cauliflower on the prepared baking sheet and season it with salt and pepper to taste. Top with another sheet of foil, roll up the edges to form a tightly sealed package.
Roast the cauliflower for 20 minutes. Remove the top layer of foil, being careful of the steam.
Continue to roast the cauliflower until it is tender and beginning to brown, about 10 minutes.
Puree roasted cauliflower and Greek yogurt until smooth, about 2 minutes. Season with salt and pepper to taste. Cover to keep warm.
Heat a large non-stick pan over medium-high heat.
Brown ground turkey, breaking into small pieces (about 4 minutes).
Drain the turkey meat and a strainer and set aside.
Reduce the heat in the pan to medium.
Add the onion to the pan and saute until tender, about 6 minutes.
Add the frozen veggies and tomatoes.
In a small bowl, whisk the chicken broth into the cornstarch. Add to vegetable mixture.
Bring the mixture to a simmer, stirring occasionally.
Reduce the heat to medium and simmer until the sauce has thickened (about 2 minutes).
Add the cooked turkey and the chives to the vegetable mixture.
Season with salt and pepper to taste.
Spread the turkey mixture in an 8 x 8-inch glass baking dish.
Spread the cauliflower puree over the turkey mixture, and sprinkle it with the whole wheat panko crumbs.
Bake until hot, about 8 minutes.