Pork sausages with cider lentils

Name: Pork sausages with cider lentils
Cuisine: European
Ingredients: 2 cups (400g) green lentils, rinsed, drained; 2 tbsp olive oil; 8 thick pork sausages; 2 onions, finely chopped; 3 garlic cloves, crushed; 1 apple, peeled, cored, cut into 1cm pieces; 1 small bunch thyme; 1 bay leaf; 1 1/3 cups (330ml) alcoholic apple cider; 1 1/3 cups (330ml) chicken stock; 1 celery stalk, finely chopped; Sour cream, to serve (optional)