Preheat your oven to 400°F (200°C).
In a medium saucepan, steam the cauliflower florets until soft (about 8-10 minutes). Once tender, mash them with a fork or blender until smooth. Season lightly with salt and pepper and set aside.
In a skillet, heat the olive oil over medium heat. Add the chopped onion and sauté for 2-3 minutes until translucent.
Add the ground turkey to the skillet and cook until it starts to brown, breaking it apart with a spatula.
Stir in the diced carrots, green peas, and tomato sauce. Sprinkle in the herbs, and season with salt and pepper. Cook for another 5-7 minutes until the vegetables are tender and the turkey is fully cooked.
Transfer the turkey and vegetable mixture into an oven-safe dish, spreading it evenly.
Spread the mashed cauliflower over the top of the turkey mixture, smoothing it out with the back of a spoon.
Place the dish in the oven and bake for 15-20 minutes until the top is slightly golden.
Remove from the oven, let it cool for a few minutes, then serve.