Place the cauliflower florets and diced potato in a medium pot, cover with water, and boil for 12-15 minutes until very tender.
Drain the vegetables thoroughly and mash them with a splash of broth or a pinch of salt until they reach a velvety, smooth consistency.
While the vegetables boil, heat the olive oil in a large skillet over medium heat and sauté the onion, carrots, and minced garlic for 5 minutes.
Finely chop the seitan to a ground-meat consistency, then add it to the skillet along with the cooked lentils, tomato paste, thyme, and rosemary.
Pour in the vegetable broth and simmer the mixture for 6-8 minutes, stirring occasionally, until the liquid reduces into a thick, savory gravy.
Fold in the frozen peas and season the filling with sea salt and black pepper, then transfer it into an 8x8 inch oven-safe baking dish.
Spread the cauliflower-potato mash over the lentil base in an even layer, using the back of a spoon to create small peaks for extra texture.
Place the dish under the oven broiler for 4-6 minutes until the top is beautifully browned and slightly crisp.