Preheat oven to 425F (220C).
Heat the oil in a large frying pan or skillet over medium-high heat. Add in the onion, carrots, mushrooms, garlic, and thyme and sauté for 6 to 8 minutes until softened and bits are browned. Remove from pan, and set aside.
Return the pan to heat, add in the lentils, and vegetable broth. Cover, and simmer for 25 - 30 minutes until the broth is absorbed and the lentils are tender. While the lentils cook, prepare the mashed potatoes according to the directions.
Add the vegetables back to the pan with the lentils, and add the peas, BBQ sauce, and soy sauce. Heat through. Scoop the mixture into a casserole dish or large skillet and then spread the mashed potatoes on top.
Bake for 10 - 15 minutes until the pie is hot and bubbling around the edges.