Steam the cauliflower florets and diced parsnips until they are very fork-tender, about 12-15 minutes.
While the vegetables steam, sauté the diced onion, carrots, and minced garlic in a large skillet with a splash of vegetable broth until softened.
Crumble the tempeh into the skillet and cook for 5 minutes until lightly browned.
Stir in the cooked lentils, tomato paste, tamari, dried thyme, and dried rosemary, mixing well to combine.
Pour in the remaining vegetable broth and simmer over medium-low heat for 8-10 minutes until the liquid has reduced into a thick, savory gravy.
Stir the frozen peas and 2 tablespoons of nutritional yeast into the lentil mixture, then remove from heat.
Mash the steamed cauliflower and parsnips with the remaining 2 tablespoons of nutritional yeast, sea salt, and black pepper until smooth and creamy.
Transfer the lentil and tempeh mixture into a small baking dish, spread the root vegetable mash evenly over the top, and broil for 3-5 minutes until the top is golden and slightly crisp.