Bring a pot of salted water to a simmer.
In a large bowl, mix together the flour, milk, and eggs and beat for 10/15 minutes until it is smooth and stretchy. Season with a pinch of salt.
Place a strainer with large holes over the pot of water. You could also use a spaetzle maker if you have one. Brush or spray with some oil. Add the dough and press it through the holes with a spatula so small pieces of dough fall into the water.
Cook the dough for 3 minutes then strain. Melt some butter in a frying pan over medium heat and add the strained spaetzle. Toss in the butter and cook for 5 minutes to lightly brown.
In a medium bowl, mix the sour cream, cheese, jalapeno, and mustard. Pour the hot spaetzle into the bowl and stir well to combine. The cheese should mostly melt and coat the spaetzle in a creamy sauce, but if it doesn't completely melt yet, that is OK. Set aside until ready to use.
Add some butter to a frying pan and add the onions. Cook over medium heat, stirring often, until browned and drastically reduced in size, about 45 minutes.
Add some butter to another frying pan. Cut the ends off the rolls so that they are the size where the ends of the sausage will stick out. Toast the bread in the pan on all 6 sides.
Add some beer to that pan, about 1 inch deep. Add the sausages and simmer about 7 minutes, flip and repeat until they are grey and cooked through. Drain the beer if there is any left and add some oil. Turn the heat up and brown the sausages.
Add a splash of milk to the bowl with the spaetzle and microwave it to warm it up. It probably will be congealed by now. Do a few increments of about 15 seconds, and add milk as needed until the mixture is hot, melty, and creamy.
Build the sausages with the bun first, then the sausage, then the spaetzle, then the caramelized onions, then finally the jalapeno.