1. Stir the rice flour, potato starch, tapioca starch, cornmeal and salt together. Whisk the eggs and the milk together and add this to the flour, stirring until blended – the mixture will be a thick paste.
2. Bring a pot of water up to a rolling boil and salt it. Spoon the spaetzle batter into a ricer (or the hopper of a spaetzle maker) and gently press this into the water, making fine, short noodles. Boil the spaetzel for about 2 minutes, or until they all float to the top of the water. Drain well. The spaetzle can be made ahead and then chilled at this point, until ready to serve.
1. To serve, heat the butter in a sauté pan over medium-high heat and add the onion, sautéing until browned, about 5 minutes. Add the spaetzle and sauté until warmed through, about 3 minutes. Season to taste and serve.