Place cucumbers in a colander over the sink or another bowl. Add salt, toss and let drain for 20 minutes. Rinse cucumbers and place in a medium heatproof bowl. Place sliced onions in a separate heatproof bowl. (Note: These can be placed in the same bowl, but the onion will turn the pickles pinkish in color. They’ll taste the same either way, though!)

Meanwhile, using a medium saucepan, add both vinegars, water, brown sugar, garlic, mustard seeds, dried dill and red pepper flakes. Bring mixture to boil. Reduce heat and let simmer for 10 minutes.

Divide the hot liquid into the two bowls, making sure the liquid completely covers the cucumbers and onions. (If necessary, add more water until the cucumbers and onions are covered.) Stir and refrigerate for 10 minutes.

Slice each pretzel bun in half.

Spread 1 Tbsp of spicy brown mustard on top half of each bun.

Layer bottom bun with Swiss cheese, ham and another slice of Swiss cheese. (Optional: toast bottom bun just until cheese melts.) Top with pickled cucumbers and onions. Place top bun on each sandwich and serve.