Preheat the oven to 350 degrees F (175 degrees C). Grease a metal colander with 1 teaspoon oil.

Whisk flour and salt together in a bowl. Beat eggs and water together in a bowl; stir into flour mixture until forms a dough.

Bring a large pot of water to a boil. Place spaetzle dough into the oiled colander; set colander over the pot of water. Push dough into the boiling water through the colander holes using a flexible spatula. Stir spaetzle as they boil until they float to the top, about 5 minutes. Transfer spaetzle into an ungreased 9x9-inch baking dish using a slotted spoon.

Cook bacon in a large skillet over medium heat, stirring often, until crisp, about 8 minutes. Transfer bacon to a paper towel; leave drippings in the skillet. Cook onion in drippings until browned, about 10 minutes; transfer to a bowl.

Melt butter in the same skillet over medium heat until foamy; stir in apple and brown sugar. Cook and stir until apple is soft, about 5 minutes. Stir bacon, onion, and sauerkraut into apple mixture; cook, stirring often, until flavors have blended, about 5 minutes. Transfer sauerkraut mixture to the baking dish on top spaetzle.

Place kielbasa pieces in a microwave-safe dish; microwave on full power until sausage is hot and lost its pink color, 2 to 3 minutes. Transfer sausage to the baking dish.

Bake in the preheated oven until bubbling, about 15 minutes.