Peel and finely dice the onions and garlic. Peel and wash carrots and celery. Clean and wash the leek. Cut prepared vegetables into fine cubes. Heat oil in a pot and fry onions and garlic until transparent.
Add carrots and celery and steam for about 3 minutes. Stir in leek and cook for about 2 minutes. Stir in tomato paste and add stock, bring to the boil and simmer for about 4 minutes. Chop lemon peel finely.
Add tomatoes and season with salt, pepper and lemon peel. Simmer for another 10 minutes. Put the pasta in plenty of boiling salted water and cook for about 10 minutes until al dente. Drain and let drain.
Wash oregano, dab dry, pluck off leaves and chop. Stir oregano into the sauce. Serve spaghetti with vegetable bolognaise. Garnish as desired with parmesan slices and oregano.