Trim veal of any large bits of sinew or fat and cut into small pieces (about 2cm). Lightly dust the veal with flour and shake off any excess.
Heat a frying pan on high, add 2 tablespoons of the olive oil and fry the veal until brown all over (be careful not to overcrowd the pan; do it in two batches if necessary). Season with salt and pepper and set aside.
Heat a heavy-based saucepan and add the remaining oil and butter. When frothing, add the onion and garlic. Cook, stirring, for a couple of minutes before adding the carrot and celery, then season with salt and freshly ground pepper. Cook for about 5 minutes on medium until soft and lightly golden. Add the browned veal, the wine, sage leaves and a quarter of a cup of water. When it comes to the boil, reduce the temperature to a simmer, cover and cook for about 35 minutes (you may need to add just a little more water if there is not enough liquid). If you are using fresh peas, pod them and add them now. Otherwise continue to cook for another 10 minutes or until the veal has become meltingly tender, adding the frozen peas for the last couple of minutes.
To serve: cook pasta in plenty of salted boiling water until al dente; drain and toss through the veal and pea ragu. Add a knob of butter and a tablespoon of grated Parmesan cheese. Serve immediately, sprinkled with a little parsley, and offer more grated Parmesan on the side.
Note: This sauce can be made one or two days ahead of time up to the point where you have added and cooked the peas. Then gently reheat and proceed as before.
If using fresh peas, ensure you either pod them yourself or use ones that have been recently podded, as they quickly become mealy.