Preheat the oven to 400. Trim and peel the sweet potatoes, beets and turnips, reserving the green tops. Cut the roots into one inch chunks.
Toss the chunks in a splash of the olive oil and sprinkle with a pinch of salt. Spread on a baking sheet. Trim & cut the onions & sweet potatoes into chunks. Toss in the olive oil, salt and spread on a baking sheet. Roast for 25-40 minutes, tossing periodically, until they are cooked through & soft to the bite.
Meanwhile, bring the lentils with three times the amount of water to a boil. Simmer for 15-25 minutes until tender but not soft. Season afterwards with salt and pepper to taste. Pour the lentils through a wire mesh strainer to remove the excess liquid.
While everything cooks, whisk together the Dijon mustard, honey, thyme, & lemon juice. Slowly drizzle in the remaining olive oil until incorporated and thick. Wash and dry the green tops. Stack up and thinly slice. Add the greens to the dressing. As the vegetables and lentils finish, toss each in the dressing while they are still hot. Taste. Add a pinch of salt, pepper and even a bit more of the thyme or lemon juice to your desired taste.
Serve hot, warm or chilled.