Preheat oven to 400°F (200°C). Coat the Jerusalem artichokes and La Ratte potatoes with olive oil. Season with salt and pepper.
Put in a baking dish and cook for 1 hour or until the vegetables are tender and golden. Set aside.
For the roasted garlic sauce: Coat the garlic in aluminum foil. Bake for 45 minutes or until the garlic is very tender. Set aside.