Cut the lamb in pieces. Melt half of the butter with the oil in a large pan where the pieces do not overlap. Before the butter browns, add the lamb, brown it very slowly so as to avoid browning the butter. Salt and pepper as soon as the lamb has the desired color, moisten with a tablespoon of cold water in order to slow the heat down. Bake in a hot oven (350F) for 20 to 30 minutes to finish the cooking. Turn the pieces several times, they must not dry out. Take out the pieces, arrange them on a serving dish, add not more than 1 or 2 tablespoons boiling water to the juice. Scrape the juices with the flat part of a fork, remove from the heat, add the remaining butter, cut into small pieces, along with the finely chopped parsley to the still boiling juice. While stirring, pour onto the lamb. Serve with diced mixed spring vegetables.
