Butter an approximately 20cm square cake tin and line the base with greaseproof paper.
Rub together the flour, oats and butter to a coarse crumb consistency. Mix in the sugar, orange zest and walnut pieces, and then press half of it into the tin.
Spread the cranberry sauce over the top and scatter over the remaining oats mixture, pressing down lightly.
Bake in the oven for 35 to 40 minutes until the top is lightly golden. Leave to cool in the tin for about 30 minutes, remove and cut into twelve squares, or larger pieces if you prefer.