olive oil1 large onion sliced (1 1/2 cups)2 cups (1/4-inch-sliced) fennel, top and core, if thick, removed3 cloves garlic, minced (1 tablespoon)1 pound sweet potatoes, peeled and sliced 1/4-inch thick1 pound celery root, peeled and sliced 1/4-inch thick1 pound yukon gold, peeled