Wash the spinach thoroughly.
In a large pan over medium-high heat, cook the spinach in 3 tablespoons (45 ml / 1.5 fl oz) of the melted butter, stirring frequently.
Continue cooking until the spinach is completely wilted and all its liquid has evaporated, about 10-15 minutes. It should just start to stick to the pan.
Transfer the cooked spinach to a blender and purée until smooth, or mince it very finely with a sharp knife.
In a large mixing bowl, combine the spinach purée, ricotta, ⅔ cup Parmesan, beaten egg yolks, 7 tablespoons (105 ml / 3.6 fl oz) of flour, salt, pepper, and nutmeg if using.
Stir vigorously with a spatula until the mixture is smooth, uniform in color, and free of lumps.
Cover the bowl and chill the mixture in the refrigerator for at least 30 minutes.
Prepare a floured surface.
Using two teaspoons, shape the chilled dough into small quenelles or ovals, about the size of a pecan. Scoop a small amount with one spoon and use the other to shape and slide it off.
Drop the formed dumpling into a small bowl of flour, gently roll to coat, shake off the excess, and place it on a wax paper-lined baking sheet.
Repeat until all the dough is used.
Bring a large pot of generously salted water (about 6 quarts) to a gentle simmer. Carefully drop the gnocchi into the water, being careful not to overcrowd the pot (cook in batches if necessary).
Simmer gently for 10-12 minutes, stirring once or twice with a wooden spoon to prevent sticking. The gnocchi are done when they float to the surface and feel slightly firm.
Remove them with a slotted spoon and drain well.
Preheat your oven to 350°F / 175°C.
Pour another 3 tablespoons (45 ml / 1.5 fl oz) of melted butter into a large, shallow gratin dish or casserole, brushing to coat the bottom evenly.
Arrange the cooked gnocchi in a single layer in the dish.
Drizzle the remaining 3 tablespoons (45 ml / 1.5 fl oz) of melted butter over the top and sprinkle evenly with the ½ cup of grated Parmesan cheese.
Bake for 15-20 minutes, until heated through and the cheese is melted and just beginning to turn golden brown.
Serve immediately.
Mix the gnocchi dough just until it comes together. Overworking it can develop gluten, resulting in tough, chewy dumplings instead of light, tender ones.
Don't overcrowd the pot.
Cook the gnocchi in batches to ensure they have enough space to cook evenly and float freely to the surface.
