Blue Cheese, Potato, and Green apple Danish Sandwich

1 lb small Yukon gold potatoes, peeled, and cut into 1/8" slices
Coarse salt
8 thin slices dense rye bread; no crusts
8 tsp unsalted butter, softened
1/2 c crumbled blue cheese
1 green apple, cored and thinly sliced
1/2 c pickled onion slices (recipe below)

Bring a large pot of water to a boil, and add potatoes, and 1 Tbs salt. Return to a boil. Reduce heat; simmer until tender, about 7 mins. Drain and let cool.

Spread bread slices with butter; sprinkle each with 1 tsp blue cheese. Top with onion slices, dividing evenly.

Sprinkle with salt and drizzle with 1 tsp pickling liquid.

Pickled Onion Slices
1/2 c white-wine vinegar
1/4 c sugar
1 bay leaf
1/4 tsp whole black peppercorns
1/2 tsp coarse salt
1 large white onion very thinly sliced

Stir vinegar, 1/4 c water, sugar, bay leaf, peppercorns, and salt in a saucepan. Bring to a boil. Add onion; return to a boil. Cook, stirring 30 seconds. Remove from heat; let cool completely. Onion slices can be stored in their liquid up to 2 weeks in air tight container in the fridge.