Preheat oven to 400 degrees. Chop potatoes and carrots into bite sized pieces and toss with olive oil, salt & pepper. Pour onto 2 baking sheets lined with foil and roast for 20-25 minutes or until slightly golden brown. Transfer potatoes to a large bowl and set aside. In a small bowl combine mayo, mustard, honey, tarragon, lemon juice & lemon zest. Whisk to combine and season with salt and pepper to taste. Pour dressing over potatoes and toss to combine. Add red onion and toss again. You can serve this dish warm or at room temperature!