For the gnocchi:
For the sauce:
1. in a medium saucepan over medium heat bring water, butter, Dijon mustard, nutmeg and salt to a boil. When the butter is melted add flour and stir with a wooden spoon until a smooth dough forms. Reduce heat to low and continue to stir. Continue cooking until dough pulls away from sides of pot.
2. Remove from heat, add the cheeses and stir until incorporated. Add eggs one at a time, allowing dough to fully incorporate egg before adding the next one (at this stage you can add herbs). Transfer the dough to a piping bag.
3. Bring a large pot of salted water to a boil. Working in batches, pipe gnocchi into the boiling water, cutting it off with a knife into 1-inch lengths and letting them fall directly into the simmering water. When the gnocchi float to the top, cook them for 2 minutes more. Lift gnocchi with a slotted spoon and transfer to a kitchen towel.
4. In a large nonstick skillet, over medium heat, melt the butter with sage leaves. When the butter is melted, and the leaves become crisp add the gnocchi. Fry until the gnocchi turn golden, about 2-3 minutes. Serve and sprinkle with parmesan.