Preheat oven to 400 degrees. Place rutabaga and celeriac in 8 x 8-inch baking dish, drizzle with 2 tablespoons olive oil and roast 20 minutes. Remove from oven and toss. Return to oven and roast an additional 20 minutes or until soft. Remove from oven and set aside.
While vegetables are roasting, rinse lentils and place in pot with vegetable stock and a pinch of salt. Bring to boil, reduce heat, cover and simmer until just tender (about 20 minutes). Remove from heat and drain well. Toss with 1 tablespoon oil and 1 tablespoon lemon juice and set aside.
In large skillet over medium heat, sauté onion in remaining tablespoon olive oil until it starts to brown (5-7 minutes). Add mushrooms and mirin and continue sautéing. Add remaining 3 tablespoons lemon juice 1 tablespoon at a time to deglaze pan and caramelize vegetables. Add thyme and sauté 2 minutes longer, for a total sauté time of 12-14 minutes. Fold in lentils and roasted vegetables and sauté to heat through. Season to taste with salt, toss with parsley and serve.
Recipe from Clean Start by Terry Walters/Sterling Epicure, 2011.