In large skillet, sauté onion and celery in 2 Tbsp olive oil until soft, about 3 to 5 minutes on medium heat. Add lentils and stir to incorporate. Increase heat; add miso, tomato paste, Worcestershire sauce, and stock. Cover and bring to boil, then reduce heat to medium-low and simmer for about 10 minutes. Add carrots and continue to cook for a further 10 to 20 minutes, until lentils are soft but there is still some liquid in the pan.

Meanwhile, in separate pan, sauté mushrooms on high heat in 1 Tbsp olive oil, stirring minimally to allow them to fully brown. Once brown, add crushed garlic and thyme and stir through for 1 to 2 minutes on medium heat.

De-glaze pan by adding wine and scraping up any brown bits from the bottom of pan. Add mushroom mixture to the lentils with 1/4 tsp salt and stir well to incorporate. Spoon lentil mushroom mixture into the bottom of a 9 x 14 in (23 x 35 cm) casserole dish.

Bring large pot of water to boil and add sweet potatoes and a pinch of salt. Boil for 10 to 15 minutes, until tender. Drain thoroughly. Add 1 Tbsp olive oil, 1/4 tsp salt, and sage; mash until smooth. Spoon sweet potatoes over lentil mushroom mixture. If you like the top crispy, roughen it up with a spoon or fork to create ridges.

Bake at 350 F for 30 minutes.