Preheat oven to 400 degrees. Clean and trim off objectionable parts, but do not peel the potatoes, then dice them into one inch cubes. Peel the garlic and slice it. Measure the olive oil into a shallow baking dish, add the potatoes and garlic and toss to coat well with oil. Place in the oven, and roast until lightly browned and tender. Meanwhile, place the peas in a colander, and peel and dice the carrots. Place the carrots in a saucepan, cover with water, and bring to a boil on the stove. Cook about 2 minutes, then pour over the peas to thaw them, rinse with cool water, and drain well. When the potatoes are done, transfer to a bowl, and let cool slightly. Combine the mustard and vinegar, and pour over the potatoes. Add the peas and carrots, salt lightly, grind fresh pepper over, and mix well. Best eaten at room temperature, but keeps for at least a week in the fridge.