Preheat the oven to 350 degrees F.
Prepare the vegetables. Cut the red potatoes in quarters. Peel and trim the baby carrots. Trim and cut the spring onions in half lengthwise. Peel and quarter the turnips.
In a medium saute pan, over high heat, add the olive oil. Add the prepared vegetables, sprig of thyme and saute for about 2 minutes. Season with salt and pepper and add butter. Continue to saute until lightly golden, another 2 to 3 minutes. Transfer to a preheated oven and roast for about 10 minutes or until caramelized.
Meanwhile, make 3 diagonal slices on each side of the trout. Season with salt and pepper inside and outside. Dredge in flour. In a large saute pan, over high heat, add 2 ounces of butter. Pan-fry the prepared trout until golden, about 5 minutes. Turn and add a sprig of sage and the remaining 1-ounce of butter. Continue to pan-fry until golden on both sides, basting the topside of the fish with the cooking butter, another 5 minutes.
Add lemon juice and parsley. Add the cooked vegetables to the trout and serve on pan for an informal al fresco dining, or on heated serving plates, place a pan-fried trout and surround with half of the caramelized vegetables. Sprinkle with parsley. Serve with lemon wedges on the side and serve immediately.