Put the 60ml / ¼ cup of olive oil in a saucepan, and mix in the garlic. Add the whole rosemary sprig, if using, and the lemon zest and warm through.
Remove the rosemary, but do not throw away.
Drain the beans and rinse under a tap to get rid of the gloop and then add to the pan and warm through, stirring and squishing with a wide, flat spoon so that the beans go into a nobbly mush. Season to taste; some beans come saltier than others.