Preheat your oven to 400°F (200°C).
In a skillet, heat the olive oil over medium heat and sauté the chopped onion, garlic, celery, and carrot until softened, about 5-7 minutes.
Add the tomato paste and a pinch each of salt, pepper, and thyme, stirring until well combined.
Stir in the cooked lentils and peas, allowing the flavors to meld for another 3-4 minutes. Adjust seasoning as needed.
Transfer the lentil and vegetable mixture to a small oven-safe baking dish.
For the creamy topping, steam or boil the cauliflower until very tender, about 8-10 minutes. Drain well.
Mash the cauliflower with the Greek yogurt, adding a little salt and pepper to taste until you achieve a smooth, creamy consistency.
Spread the cauliflower mixture evenly over the lentil base in the baking dish.
Place the dish in the preheated oven and bake for 15 minutes, until the topping is slightly golden and the filling is bubbly.
Remove from the oven and let sit for a few minutes before serving to allow the layers to set.