Preheat oven to 350 degrees F (175 degrees C). Grease a metal colander.
Whisk together the flour and salt in a bowl.
Mix the eggs and water, and pour into the flour mixture. Stir to form a dough.
Bring a pot of water to a boil.
Place the spaetzle dough into the oiled colander, and set the colander over the pot of water. Use a flexible spatula to push the dough into the boiling water through the holes in the colander. Stir the spaetzle as they boil. When they have floated to the top, they are done (about 5 minutes). Scoop the spaetzle out of the boiling water, and place into an ungreased 9x9-inch baking dish.
Place the bacon in a skillet over medium heat, and cook, stirring often, until the bacon is crisp, about 8 minutes.
Remove bacon, leaving drippings in pan, and set aside. Cook the onion in the same pan as the bacon until browned, about 10 minutes.
In a large skillet, melt the butter over medium heat until foamy, and stir in the apple and brown sugar. Cook and stir until the apple is soft, about 5 minutes.
Transfer the cooked bacon, onions, and sauerkraut into the skillet, and cook, stirring often, until the flavors have blended, about 5 minutes.
Pour the sauerkraut mixture into the 9x9-inch baking dish on top of the spaetzle.
Place the kielbasa pieces into a microwave-safe dish, and cook on full power in a microwave oven until the sausage is hot and has lost its pink color, 2 or 3 minutes.
Transfer the sausage to the baking dish.
Bake in the preheated oven until bubbling, about 15 minutes.
