In a large sauté pan (or other flat-bottomed pan) over medium heat, heat oil and add leeks, celery, carrot and oregano or marjoram.
Sauté until leeks are translucent, about 5 minutes. Add rice and sauté 4 minutes, to coat with oil.
Pour in 3 cups of stock, add saffron and stir until mixture begins to simmer. Add corn, pepper, zucchini, tomato and another 3 cups of stock.
Once simmering again, add clams and cook until they begin to open. Add mussels, and again, simmer until they begin to open, adding more stock if needed.
Add white fish and shrimp and stir until shrimp turn pink.
Check that rice is tender (and cook a bit longer if needed), then add lemon juice, dill and season to taste. Serve immediately.