Peel and crush the garlic. Rinse salt from capers under running water. Chop olives; rinse and chop parsley. In a saucepan heat the oil with the garlic and remove once golden. Add the capers and olives, flavor for a few minutes and remove from the heat. Cook the spaghetti in well salted boiling water and drain when al dente. Pour into bowl and mix with the capers and olives. In another bowl, whisk the eggs and the parsley with a fork and season with a pinch of salt and a grind of pepper. Combine the spaghetti with the egg and parsley sauce and mix well with a wooden spoon. Heat butter in an oven proof dish, then put spaghetti mixture and bake in preheated oven at 250 °C or, even better, under the grill for about 5 minutes, until the top of the pasta is golden brown. Remove the dish from the oven, let it cool for a minute and serve warm.