Preheat your oven to 400°F (200°C).
Place the cauliflower florets and diced potato in a pot of boiling water and cook until very tender, about 12-15 minutes.
While the vegetables boil, heat the olive oil in a large skillet over medium heat and sauté the onion, carrots, and garlic until softened.
Stir the tomato paste, dried thyme, and dried rosemary into the skillet, cooking for 1 minute to release the aromatics.
Add the cooked lentils, frozen peas, and vegetable broth to the skillet; season with half the salt and pepper, then simmer for 5 minutes until the liquid is slightly reduced.
Drain the boiled cauliflower and potato, then mash them thoroughly with the Greek yogurt, nutritional yeast, and the remaining salt and pepper until smooth.
Transfer the lentil and vegetable mixture into a small baking dish, spreading it into an even layer.
Spread the cauliflower-potato mash over the top of the lentils, using a fork to create small peaks for extra browning.
Bake for 20 minutes, or until the filling is bubbling and the topping has developed a beautiful golden-brown crust.