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 desired amount of vegan butter (3-4 Tbsp as original recipe is written // adjust if altering batch size), and season with salt and pepper to taste. Loosely cover and set aside.
While potatoes are cooking, preheat oven to 425 degrees F (218 C) and lightly grease a 2-quart baking dish (or comparable sized dish, such as 9x13 pan. An 8x8 won’t fit it all but close! // as original recipe is written // adjust number or size of dish if altering batch size).
In a large saucepan over medium heat, sauté onions and garlic in olive oil until lightly browned and caramelized - about 5 minutes.
Add tomato paste (optional) and a pinch each salt and pepper. Then add lentils, stock, and thyme and stir. Bring to a low boil. Then cover and reduce heat to simmer. Continue cooking until lentils are tender (35-40 minutes). Once 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, stir, and cover to meld the flavors together.
OPTIONAL: To thicken the mixture, add 2-3 Tbsp (amount as original recipe is written // adjust if altering batch size) mashed potatoes and stir. Alternatively, scoop out 1/2 of the mixture and whisk in 2 Tbsp (amount as original recipe is written // adjust if altering batch size) 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 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.
*Inspiration from Martha Stewart *Nutrition information is a rough estimate.