Heat the oven to 170 fan. Butter or lightly oil a 9x5 inch/ 22x12 cm loaf tin.
Put the courgettes into a bowl with the eggs, oil, butter, sugar, salt, nutmeg, lemon zest and rosemary. Mix all this together then add the baking powder, baking soda, flour and walnuts. Scrape this into the prepared tin and smooth the top. Lay the sprigs of rosemary on the surface.
Bake for 50-60 minutes. Put a skewer into the middle of the cake – if the cake's ready the skewer should come out clean. Leave to cool in the tin then carefully turn it out.
To make the butter toast the coriander, fennel and peppercorns in a dry pan until you can smell them (about 20 seconds). Tip them into a mortar and pestle and crush them. Add the chilli flakes. Work these spices and the sesame seeds into the soft butter, adding honey as you go. Scrape into a small bowl or ramekin, cover and put in the fridge to firm up. Serve with the bread.
