In a large braising pan, sauté the onion very gently in the oil and butter until it softens, adding a spoonful of water while it cooks to make sure it doesn't brown. Turn up the heat to medium and add the veal, turning and mixing it with the onion, until the veal has lost its raw color completely and browned just a bit at the edges. (Adjust the heat if needed to avoid the onion burning.) Season with salt and pepper, turn again, and add a splash of white wine and let it cook down.

Add the tomatoes, turn again, and let things simmer for a few minutes. Now add enough water or broth to almost cover the veal and nestle the sprigs of mint among the veal cubes. Cover the pan, lower the heat as far as it will go, and let the veal simmer until it is fully tender, about 1-1/2 hours or so, depending on the age of the veal and the size of your cubes. Add water from time to time if things get a bit dry.

Meanwhile, if using fresh peas, shell them. Sauté the onion in butter as above until very soft and sweet. Add the shelled peas and let them simmer to absorb the flavors of onion and butter, then add enough water or broth to cover. Simmer the peas over moderate heat until the liquid has entirely cooked off. The peas should be mostly, but not quite entirely, done.

About ten minutes or so before the veal is done, add the simmered fresh peas (or the frozen peas) to the braiser. Let the peas and veal simmer together until both are fully tender. Once again, add a bit of water if things get too dry. The resulting sauce should, however, be quite thick and rich.

Serve right away.