Mashed Cauliflower: Steam the cauliflower for 10-15 minutes until tender. Let cool.
Blend cauliflower in a food processor with olive oil or Earth Balance, salt, and pepper. Set aside.
Preheat oven to 425°.
Filling: Heat olive oil in a pan. Add chopped onion, carrot, and celery and cook until the vegetables are soft, about 8-10 minutes.
Add garlic to the pan. Cook for a minute longer.
Add chopped baby portobello mushrooms, red wine, and thyme. Cook until the red wine cooks down to about half the amount.
Add vegetable broth, tomato paste, lentils, tamari, salt, and pepper. Cook for 10 minutes, then add the peas. Cook another 5-10 minutes, stirring constantly, until lentils are soft and tender. Add more broth if needed.
Pour lentil and mushroom mixture into a 9 ½" x 7 ½" (or similar size) baking dish. Spread mashed cauliflower on top. Bake at 425° for 10-15 minutes, then broil for 5 minutes to brown the cauliflower.