Peel potatoes. Place potatoes, in one layer, in a large (12-inch) skillet with lid. The potatoes must not be on top of each other. Add only about 1 inch of water and ½ teaspoon of salt to pan. The water will barely come halfway up the potatoes. Bring to a boil, then turn heat to very low. Cover pan tightly and simmer gently until potatoes are just tender when pierced with a knife, about 30 to 35 minutes. Check occasionally to make sure the heat is not too high and the water has not boiled away. Add more water if needed. The potatoes are basically steamed so they retain their shape in order to slice nicely. When done, drain potatoes and set aside.
Heat oven to 350 degrees.
Using the same skillet, fry the bacon until crisp. Remove with a slotted spoon and drain on paper towels; set aside. To the drippings in pan, saute the onion and celery until tender. Stir in the flour and salt. Cook over low heat, stirring constantly, until bubbly. Remove from heat and stir in the water, vinegar, and sugar. Return to heat, and heat to boiling, stirring constantly. Boil and stir for 1 minute. Stir in bacon and parsley; remove from heat.
Cut potatoes into slices and stir gently into mixture in pan; mix well. Transfer potato mixture to a 3-1/2 to 4-quart casserole dish. Cover dish and bake at 350 degrees for 30 minutes. Remove from oven and stir in the radishes. Serve immediately, sprinkled with more chopped parsley if desired. Makes 12 servings.
Recipe by The Culinary Cellar at https://theculinarycellar.com/oktoberfest-hot-german-potato-salad/