Boil the trimmed green beans in well salted water until tender. Drain, run briefly under cold water and let cool. When the green beans have fully cooled, cut them into very short lengths, about as long as they are wide.
Boil the potatoes, unpeeled, until soft. Drain, run under cold water and let them cool. When they have cooled off enough to handle, peel them with a paring knife and purée them in a food mill, using the large holes, over a large mixing bowl.
Add the chopped up green beans to the bowl, along with the eggs, parmigiano-reggiano, marjoram, salt and pepper and nutmeg if using. Mix everything together well until well amalgamated. Taste and adjust for seasoning.
Take a loaf pan and grease the inside well with olive oil or butter. Coat generously with breadcrumbs.
Pour the green bean and potato mixture into the pan, leveling it out with a spatula. Top with breadcrumbs and drizzle with olive oil or dot with butter.
Bake in a moderately hot (190C/375F) oven for 30-45 minutes, until cooked through and nicely browned on top. (To test doneness, insert a paring knife to see if it comes out clean.)
Remove the pan from the oven and let cool to just slightly warm or (even better) room temperature. Unmold onto an oval dish and serve in slices.