Lentil Soup with Spinach & Sausage

1 medium onion, chopped
1/4 cup olive oil
2 carrots, diced
2 stalks celery, chopped
2 cloves garlic, minced
1 teaspoon dried oregano
1 bay leaf
1 teaspoon dried basil
1 (14.5 ounce) can crushed tomatoes
2 cups dry lentils (I prefer du puy)
8 cups chicken stock
2-3 generous handfuls fresh baby spinach
3-4 chicken Italian sausages, cooked and sliced into rounds
sea salt
fresh black pepper

In a large stock pot, heat oil over medium heat. Add onions, carrots, and celery; cook and stir until onion is tender. Stir in garlic, bay leaf, oregano, and basil; cook for 2 minutes.
Stir in lentils, and add chicken stock,tomatoes and sausage. Bring to a boil. Reduce heat, and simmer for at least 1 hour. When ready to serve stir in spinach, and cook until it wilts. Season to taste with salt and pepper.