Place sweet potatoes in a large pot and fill with water until they’re just covered. Bring to a boil over high heat. Then reduce to medium-high heat and cook for 15-20 minutes or until they slide off a knife very easily.
Once cooked, drain and transfer to a large mixing bowl. Use a masher or fork to mash until smooth (see photo). Add coconut oil and season with salt and pepper to taste. Depending on the sweetness of sweet potatoes, add a bit of maple syrup to taste (optional). Loosely cover and set aside.
While potatoes are cooking, preheat oven to 425 degrees F (220 C) and lightly grease a 9x13-inch (or similar size) baking dish (or more baking dishes, as needed, if altering batch size).
If adding (optional), add portobello mushrooms to a shallow dish or large Ziploc bag. Top with balsamic vinegar, oil (optional), salt, pepper, and garlic. Toss to combine and let marinate for 5 minutes.
Next heat a large pot over medium heat. Once hot, add oil, onions, and garlic and sauté until lightly browned and caramelized - about 4-5 minutes.
Add a pinch each salt and pepper. Then add lentils, vegetable broth, and thyme and stir. Bring to a low boil. Then reduce heat to simmer. Continue cooking until lentils are tender (20-30 minutes).
(If adding mushrooms) while lentils cook, heat a large skillet over medium heat. Once hot, add a dash of oil and the mushrooms with their marinade. Spread into an even layer and cook on one side for 2-3 minutes. Then carefully flip and cook on other side for 2-3 minutes. Then remove from heat and set aside.
In the last 10 minutes of cooking the lentils, add the frozen veggies, stir, and cover to meld the flavors together. Drain off any extra vegetable stock (there shouldn't be much). Taste and adjust flavor as needed, adding more salt and pepper for overall flavor.
Transfer the lentil mixture to your oven-safe baking dish and top with portobello mushrooms (optional) then mashed sweet potatoes. Smooth down with a spoon or fork and season with another crack of pepper and a little sea salt (and more thyme if desired).
Set on a baking sheet to catch any overflow and bake for 20 minutes or until the potatoes are lightly browned on top and the edges are bubbling.
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 up to 4-5 days or in the freezer up to 1 month. Reheats well in the microwave or in the oven.
*3 large sweet potatoes is equal to ~5 small sweet potatoes. *You can sub 1 tsp dried thyme per 2 tsp fresh thyme. *Nutrition information is a rough estimate calculated with 3 Tbsp of oil total, and without the baked mushrooms or optional maple syrup. *Recipe adapted from my 1-Hour Vegan Shepherd's Pie.