Pre-heat the oven to 425 degrees. Chop the sweet potatoes into 1 inch pieces. Bring 2 cups of water to a boil over high heat and add the sweet potatoes. Cook until they are easily pierced with a fork, about 20 minutes.

Drain the water and use a fork to mash the potatoes. When they are almost completely mashed, add the butter and a sprinkle of salt and pepper. Continue mashing until smooth.

In a sauce pan, combine the lentils, frozen vegetables and broth. Cover and cook for 20 minutes, until lentils are cooked and liquid has reduced by about 50 percent. Add 2 tbsp of sweet potato to the pot and stir to thicken the mixture.

Grease an 8x8 baking pan and spoon the lentil mixture in. Spread the potatoes over the top to cover the lentils, then bake for 20 minutes.

Cool slightly before serving.