Preheat oven to 250C or onto the grill setting.
In a processor, add the spinach, courgettes, oregano, garlic, lemon zest, juice, capers and walnuts then blend until finely chopped.
Stir in the olive oil and season generously with salt and pepper.
Stuff the mixture into the belly cavity of the sardines then place into a baking dish.
Season the sardines with more salt and pepper then dot the butter on top.
Bake in the oven for 8 – 10 minutes or until the sardines have crisped up and the fish is cooked through.
Serve with a crisp side salad.