Pluck rosemary needles from the stem. Dab the fillet dry and cut into small medallions. Put 3-4 medallions on a skewer. Season with salt, pepper and rosemary. Heat the fat in a pan.
Fry the skewers for about 10 minutes while turning. Clean, wash and halve the plums. Remove stones, cut the flesh into slices. Peel the shallots and, depending on the size, halve or quarter them
Remove skewers and keep warm. Steam shallots in the frying fat until translucent. Add plums and green pepper. Add vinegar and 100 ml water.
Season with salt, pepper and sugar and cook for about 10 minutes. Bring the sauce to the boil again, add the sauce thickener while stirring. Serve skewers with the plum sauce. Delicious with baguette