Heat olive oil in a saucepan and sauté finely chopped onion until transparent. Add Italian peeled tomatoes and season to taste with salt, freshly ground black pepper and oregano. Bring to a boil and cook over a medium heat for 15 minutes.
Cook tagliatelle in boiling salted water until al dente - tender but still firm. Drain.
Melt butter in a heatproof casserole. Ad