Wash and dry all produce. Bring a large pot of salted water to a boil. Halve, peel, and dice onion. Peel and finely dice carrots. Mince or grate garlic. Trim, then roughly chop mushrooms. Roughly chop parsley. Remove sausage from casings.

Heat 4 tsp olive oil in a large pan over medium heat. Add onion, carrots, and garlic and cook until softened, about 5 minutes. Toss in mushrooms and cook until tender, about 5 minutes. Season with salt and pepper.

Once water is boiling, add spaghetti to pot. Cook until al dente, 9-11 minutes. Scoop out and reserve 1 cup pasta cooking water, then drain. Once mushrooms are tender, add sausage to pan, breaking up meat into pieces. Cook until no longer pink, about 5 minutes.

Stir crushed tomatoes, pasta cooking water, and stock concentrate into pan. Simmer until slightly thickened, 4-5 minutes. Season with salt and pepper.

Stir spinach into pan with Bolognese and let wilt. (TIP: If Bolognese is not saucy or seems stiff, add an extra splash of water.) Taste and season with salt and pepper.

Divide spaghetti between plates and top with Bolognese. Sprinkle with Parmesan and parsley.