Lentils with Chicken Sausage

Name: Lentils with Chicken Sausage
Ingredients: 1 cup French green lentils (7 ounces); 4 cups chicken stock or low-sodium broth; 1 carrot, cut into 1-inch pieces; 1 celery rib, cut into 1-inch pieces; 1 small onion, quartered, plus 1 large onion, cut through the root end into 8 wedges; 1 garlic clove; 2 tablespoons canola oil; 4 thyme sprigs; 1 cup baby spinach leaves, coarsely chopped; Salt and freshly ground pepper; 4 precooked chicken sausages (3/4 pound); 1 1/2 tablespoons balsamic vinegar