Place terrine mould or loaf pan in refrigerator to chill and preheat oven to 350°F (180°C). Melt butter in a large saucepan over medium heat. Raise the heat to medium-high, and sauté spinach and mint leaves. Cook just until wilted and allow to cool. Mix Ricotta with eggs and yolks until smooth. Stir in the flour, grated Parmesan and nutmeg. Fold in wilted spinach and mint and season to taste with salt and pepper. Brush terrine mould with melted butter. Pour the mixture into the chilled loaf tin and cover with aluminum foil. Bake in the oven at 180`c for 30 minutes then remove the foil and continue to cook for about 15 minutes or until a toothpick inserted in centre comes out clean. Remove from oven and let cool before slicing.