Delicious sausage & lentil casserole

Name: Delicious sausage & lentil casserole
Prep time: 15 min
Cook time: 1 h 30 min
Total time: 1 h 45 min
Cuisine: European, British
Ingredients: 6 fat sausages (read above for my suggestions!); 2 onions (finely chopped); 2 sticks celery (finely chopped); 2 carrots (peeled and finely chopped); 1 parsnip (peeled and finely chopped); 750 ml vegetable stock (I use the Marigold Bouillon whichin my opinion is the best- read above!); 1 tin chopped tomatoes; 250 g green lentils (dry); 1 tsp dried parsley; Salt and pepper; Low calorie cooking spray; Baby leaf spinach (to serve)