Use a 1.2-litre ovenproof dish. Preheat oven to 180 degrees C. Put the plums, orange zest, juice, cinnamon stick and sugar in the dish and mix well. Gently shake the dish so the fruit settles evenly.
To make the topping, sift flour and baking powder in a mixing bowl. Rub in butter until mixture resembles fine breadcrumbs. Stir in the sugar and chopped walnuts. Make a well in the centre, add milk and mix to a soft (not sticky) dough.
Transfer the dough to a lightly floured work surface and knead briefly. Roll out to a rectangle about 1cm thick and a little wider than the diameter of the dish. With a sharp knife or pastry wheel, cut dough into strips 2 cm wide.
Dampen the rim of the dish with water. Arrange the strips over the fruit in a lattice pattern, pressing each end onto the rim of the dish and trimming the ends neatly. Brush the lattice with the extra milk.
Bake for 25–30 minutes or until the topping is golden and the fruit is tender. Serve hot or warm. Remove cinnamon before serving.