Preheat the oven to 400°F.
In a skillet over medium heat, warm the olive oil and add the chopped onion, sautéing until soft and translucent.
Add the ground turkey to the skillet, breaking it apart with a spoon. Season with salt, pepper, dried thyme, and rosemary. Cook until the turkey is browned and nearly cooked through.
Stir in the chopped carrot and green peas, letting the vegetables soften slightly in the turkey mixture for about 5 minutes. Adjust seasoning as desired.
Meanwhile, steam or boil the cauliflower florets until very tender, about 8-10 minutes.
Drain the cauliflower and mash it until smooth using a fork or an immersion blender. Season lightly with salt and pepper.
Spread the turkey and vegetable mixture in an ovenproof dish. Gently layer the mashed cauliflower on top, smoothing out the surface.
Bake in the preheated oven for about 10-15 minutes until the top is slightly golden and everything is heated through.
Remove from the oven, let it cool for a few minutes, then serve warm.