Preheat oven to 350 degrees.

Place potatoes in a large pot, cover with cold water, and add salt.

Bring to a boil and cook until potatoes are just tender, about 5 minutes.

Drain the potatoes and set aside.

Cook bacon in a large skillet over medium heat until crispy, about 8 minutes. Remove bacon and set aside.

Measure the bacon fat in the skillet, adding olive oil if needed to equal 1/4 cup.

Return the skillet to medium heat and add celery and onion.

Cook, stirring, for 1 minute.

Stir in flour until incorporated.

Slowly add 1 1/3 cups water and vinegar, stirring constantly, until the mixture thickens, 2-3 minutes.

Remove the skillet from heat and stir in sugar, salt, celery seed, mustard, pepper, and parsley.

Pour the sauce over the potatoes and bacon, tossing gently to coat.

Transfer the potato mixture to a 3-quart baking dish.

Cover with foil and bake for 35 minutes.

Let stand for 10 minutes before serving.