Prepare the Cauliflower: Steam the cauliflower florets in a pot until tender, about 10-15 minutes. Drain and mash with milk, salt, and pepper until smooth.
Cook the Filling: In a large skillet, heat olive oil over medium heat. Add the diced onion and garlic, sautéing until translucent. Add the carrots, peas, lentils, tomato paste, thyme, rosemary, and vegetable broth. Cook until heated through, about 5-7 minutes.
Assemble the Pie: Preheat the oven to 400°F (200°C). In a baking dish, spread the lentil mixture evenly, then top with the cauliflower mash. Smooth the top and sprinkle with cheese if desired.
Bake: Place in the preheated oven and bake for 20-25 minutes, or until the top is golden and bubbly.
Serve: Allow to cool slightly before serving. Enjoy your hearty and delicious cauliflower shepherd's pie!