Spaghetti with Chicken Sausage and Lentil Bolognese

Name: Spaghetti with Chicken Sausage and Lentil Bolognese
Ingredients: 1 tablespoon extra virgin olive oil; 1 onion (finely chopped); 3 stalks celery (diced); 3 cloves garlic (minced); 1 lb chicken sausage (casing removed); 1 bay leaf; 1/2 cup beef stock; 28 oz strained tomatoes ((aka passata or tomato purée)); 1 tablespoon tomato paste; 2 cups cooked lentils; 1 lb spaghetti; Kosher salt (to taste); Freshly ground black pepper (to taste); Freshly grated Parmesan cheese (for serving); 1/4 cup chopped flat-leaf parsley (for serving)