To make the dressing, whisk the garlic with the mustard and vinegar. Add about 150 ml vegie oil, little by little, continually whisking. Then season and set aside. Heat a little fresh oil and the butter in a heavy-bottomed pan and gently sauté the leeks until tender, stirring now and again and adding a little water if sticking. Add the cannellini beans, season quite well and toss to heat through. Then remove from the heat and add the parsley with the dressing to taste. Toss well and serve