In a medium bowl, whisk together the flour, egg, water, nutmeg, and a pinch of salt until a thick, smooth batter forms; let the batter rest for 10 minutes.
Season the chicken breast with half of the sea salt and black pepper.
Heat avocado oil in a skillet over medium-high heat and sear the chicken for 6-7 minutes per side until golden brown and the internal temperature reaches 165°F.
Bring a large pot of salted water to a boil, then press the spätzle batter through a spätzle maker or a colander with large holes directly into the boiling water.
Cook the spätzle for 2-3 minutes until they float to the surface, then drain them immediately.
In the same skillet used for the chicken (after removing the meat to rest), melt the butter over medium heat until it begins to foam and turn a light amber color.
Add the fresh sage leaves to the butter and fry for 30-60 seconds until crisp.
Add the drained spätzle to the skillet, tossing well to coat in the brown butter and sage.
Slice the chicken and serve it over the warm, buttery spätzle.