Preheat oven to 425 degrees F and lightly grease a 9×13 pan or similar sized dish.
Line a baking sheet with parchment and set aside for mushrooms.
Slice or quarter mushrooms and then with olive oil and season with salt and pepper. Place on the baking sheet (don't crowd!) and let roast for 15 minutes. Once done, set aside.
While the mushrooms are roasting, sauté onions and garlic in a large saucepan in olive oil over medium heat,until lightly browned.
Add tomato paste, lentils, stock, thyme, salt, pepper, sherry and stir. Bring to a low boil, cover and reduce heat to simmer. Cook lentils until they are tender (35-40 minutes). When tender, remove the lid and continue simmering uncovered, stirring frequently, to evaporate any excess liquid.
In the last 10 minutes of cooking, add the frozen veggies and stir. Leave the top off to make sure there's no excess liquid.
When lentils and vegetables are done, adjust seasoning if necessary. Then transfer to your baking dish and top with the mashed potatoes. Smooth down with a spoon or fork and top with a little freshly cracked pepper and a sprinkle of salt.
Bake for 10-15 minutes, until the potatoes are lightly browned on top.
Let cool briefly to thicken before serving. For color, sprinkle a few chopped chives on top.