Melt the butter in a nonstick frying pan. As soon as it stops foaming, add the sage leaves and fry over a medium-high heat, stirring, for 2–3 minutes until crisp and the butter turns golden brown. Take out 6 of the leaves and drain on kitchen paper. Transfer the remaining leaves and butter to a bowl.
Beat the goats' cheese and crème fraîche together in a separate bowl. Beat the eggs in another bowl, season with salt and pepper, then stir in the sage leaves with the butter.
Reheat the frying pan, adding a little extra butter if necessary. Pour in the egg mixture and dot over spoonfuls of the goats' cheese mixture. Cook over a medium heat for 4–5 minutes until the underside is set, then transfer to a preheated hot grill to brown the top lightly. Leave to cool slightly, then gently slide the frittata on to a serving plate. Garnish with the reserved sage leaves and serve with crusty bread.