Instructions

Whisk sorghum flour, all-purpose flour, ½ teaspoon pepper, and ½ teaspoon salt together in large bowl. Whisk milk and eggs together in second bowl. Slowly whisk milk mixture into flour mixture until smooth. Cover and let rest for 15 to 30 minutes. Meanwhile, use skewer to poke about forty ¼-inch-wide holes in bottom of disposable pan, moving skewer in circular motions to widen holes as needed. Combine fontina and Gruyère in third bowl. 
Bring 4 quarts water to boil in Dutch oven. Add 1 tablespoon salt to boiling water and set prepared disposable pan on top of Dutch oven. Transfer half of batter to disposable pan. Use spatula to scrape batter across holes, letting batter fall into water and stirring to prevent clumping. Boil until all spätzle float, about 30 seconds. Using spider skimmer or slotted spoon, transfer spätzle to colander set in large bowl to drain. Repeat with remaining batter. Toss drained spätzle with butter and set aside. Drain leftover cooking water in Dutch oven and wipe pot dry with paper towel. 
Adjust oven rack 6 inches from broiler and heat broiler. Heat 1 tablespoon oil in now-empty Dutch oven over medium heat until just smoking. Add kielbasa and cook, stirring occasionally, until spotty brown, about 5 minutes. Using slotted spoon, transfer kielbasa to bowl with spätzle. 
Add onion and remaining ¼ teaspoon salt to fat left in pot and cook, stirring frequently, until onion is golden brown, about 15 minutes, adjusting heat and adding water 1 tablespoon at a time if onion or bottom of pot becomes too dark. 
Off heat, stir in reserved spätzle-kielbasa mixture, ¾ cup cheese mixture, sauerkraut, mustard, caraway seeds, and remaining ½ teaspoon pepper. Sprinkle remaining cheese mixture evenly over top. Broil until bubbling and cheese is browned in spots, about 5 minutes. Let cool for 10 minutes. Sprinkle with dill. Serve.