Preheat the oven to 450 degrees F (230 degrees C).
Heat olive oil in a large skillet over medium heat. Add cauliflower and saute, stirring often to prevent it from getting too brown, until soft enough to mash with a fork, 8 to 10 minutes. Transfer to a large bowl, add Greek yogurt, and mash with a fork. Pour in chicken broth a little at a time, continuing to mash until fluffy and smooth. Season with salt and pepper.
Cook ground turkey and garlic in a large nonstick skillet over medium-high heat until browned and crumbly, 5 to 7 minutes. Drain and discard grease. Stir in soup mix and flour until combined, then slowly add water and stir until creamy but not runny. Simmer until thickened, 2 to 3 minutes.
Spread turkey mixture in the bottom of a 9x13-inch casserole dish. Layer with frozen corn, cauliflower mixture, and Cheddar cheese.
Bake in the preheated oven for 15 minutes. Sprinkle with french-fried onions and bake until browned, about 5 minutes more. Remove from the oven and allow to sit for a few minutes before serving.