Preheat your oven to 350°F (175°C) and grease an 8x8-inch (or 9x13-inch) casserole dish with olive oil.

In a large pot, bring water to a boil and add peeled, chopped russet potatoes along with cauliflower florets. Cook for about 10 minutes until both are soft and fork-tender. Drain the mixture well, then mash it together with olive oil, garlic powder, black pepper, salt, chicken broth, and grated Parmesan until smooth.

In a skillet over medium heat, warm a tablespoon of olive oil and add the ground turkey. Cook until browned and fully cooked through, stirring frequently for about 8-10 minutes. Add diced onions, minced garlic, and diced carrots, sautéing until the vegetables soften, about 5 minutes.

Stir in salt, pepper, dried rosemary, thyme, along with low sodium chicken broth and tomato paste to the turkey mixture. Add in frozen peas and corn, mixing everything until well combined. Simmer for 2-3 minutes to allow flavors to meld together.

Spread the turkey filling evenly across the bottom of your greased casserole dish. Layer the creamy potato-cauliflower mash on top, smoothing it out with a spatula.

Place the assembled casserole in the preheated oven and bake for 30 minutes until the topping is golden brown.