Slice any large potatoes in half, place in a large pot and fill with water until they’re just covered. Bring to a low boil on medium high heat, then generously salt, cover and cook for 20-30 minutes or until they slide off a knife very easily.
Once cooked, drain, add back to the pot to evaporate any remaining water, then transfer to a mixing bowl. Use a masher, pastry cutter or large fork to mash until smooth. Add add desired amount of vegan butter (2-4 Tbsp), and season with salt and pepper to taste. Loosely cover and set aside.
While potatoes are cooking, preheat oven to 425 degrees F and lightly grease a 2-quart baking dish (or comparable sized dish, such as 9x13 pan).
In a large saucepan over medium heat, sauté onions and garlic in 1 Tbsp olive oil until lightly browned and caramelized - about 5 minutes.
Add a pinch each salt and pepper. Then add lentils, stock, thyme and stir. Bring to a low boil, then reduce heat to simmer. Continue cooking until lentils are tender (35-40 minutes).
In the last 10 minutes of cooking, add the frozen veggies, stir, and cover to meld the flavours together.
OPTIONAL: To thicken the mixture, add 2-3 Tbsp mashed potatoes and stir. Alternatively, scoop out 1/2 of the mixture and whisk in 2 Tbsp cornstarch or arrowroot powder and whisk. Return to the pan and whisk to thicken.
Taste and adjust seasonings as needed. Then transfer to your prepared oven-safe baking dish and carefully top with mashed potatoes. Smooth down with a spoon or fork and season with another crack of pepper and a little sea salt.
Place on a baking sheet to catch overflow and bake at 425 for 10-15 minutes, or until the mashers are lightly browned on top.
Let cool briefly before serving. The longer it sits, the more it will thicken. Let cool completely before covering, and then store in the fridge for up to a few days. Reheats well in the microwave.