Preheat the oven to 400° degrees.
Add the Brussels sprouts, sweet potato, walnut oil, and seasoning to a casserole dish (8" x 8" or 9" x 13" will both work). Toss everything together to coat the veggies.
Place on the center rack of the oven and bake 45 minutes, stirring once half-way through, or until veggies are golden-brown and cooked through. Brussels sprouts generally take 30 - 40 minutes, sweet potatoes 45 minutes.
In a small saucepan, heat the balsamic vinegar over medium-high and bring to a full boil. Cook at a full (but controlled) boil, stirring frequently until vinegar is reduced by half of its original volume, about 5 minutes.
To serve: Plate the roasted veggies and sprinkle with the pecans and feta, and drizzle the balsamic reduction on top.