For pasta, blanch fresh spinach in salted boiling water for 1 minute. Drain and squeeze out excess moisture (if using frozen, thaw completely and squeeze out water). Place spinach in a food processor and puree until smooth. Add flour, salt and eggs and pulse until mixture forms a tight ball. Wrap dough in plastic and chill for at least 2 hours.
While dough is chilling, prepare filling. Stir all ingredients together and chill.
To assemble, roll out pasta, using a pasta machine into thin sheets. Cut 16 squares from the pasta, the width of the pasta sheet itself. Lay out 8 squares and spoon filling equally into the center of each square. Brush edges around filling with water and top with remaining pasta squares. Press filling down a little, to spread it, then gently press pasta edges together, starting close to the filling and working outwards (to push out any air bubbles). Chill ravioli until ready to cook and serve.
For sauce, melt butter in a large sauté pan with sprigs of sage and cook over medium heat, just until foaming subsides. Set aside until ready to serve. Bring a large pot of salted water up to a boil and gently drop in ravioli (4 at a time). Remove ravioli with a slotted spoon into pan with butter and warm over medium-low. Transfer ravioli to serving plates and spoon with any remaining butter. Sprinkle with chopped sage and mint, season lightly and top with grated Parmesan.