Thinly slice the peaches. I recommend leaving the skin on so that the peaches keep their structure during grilling. Thinly slice the halloumi. Set aside.
Slice the cucumber in half from tip to tip. Scrape out the seeds. Slice each half into ¼-inch pieces. On a cutting board or in a wide-bottomed bowl, mash the cucumber with a potato masher or heavy-bottomed glass so that they are tenderized and slightly broken down.
Add the rice vinegar, chili garlic sauce, honey, and salt. Toss to combine. Taste and adjust as needed, adding extra chili garlic sauce for more spice or more rice vinegar for extra acidity. Set aside.
Heat a grill pan or grill over medium-high heat. Once hot, add the peaches and halloumi. Cook on each side until charred, just a few minutes. I find tongs work best to flip the peaches and halloumi.
Drizzle the ciabatta slices with olive oil. Toast on the grill pan for 2-3 minutes, until golden.
(Optional) Spread mayonnaise on one piece of ciabatta. Use a slotted spoon or fork to top with cucumber salad, then layer with peaches, halloumi, and arugula. Best enjoyed immediately.
Notes
Halloumi: I find this in the specialty cheese section by paneer and feta. It's sold in block form.
This recipe makes 4 servings of cucumber salad. It's great enjoyed on its own or as a side with the halloumi sandwiches.
Cucumber salad: This is inspired by oi muchim (Korean cucumber salad), but it is not at all meant to be a traditional recreation. Oi muchim typically uses gochugaru (Korean red chili flakes) and I'm using chili garlic sauce by Huy Fong which is Vietnamese. Sambal oelek and sriracha are good swaps if you don't have chili garlic sauce but want to keep the spicy aspect.
No grill pan or grill? No problem. Just cook the peaches and halloumi in a hot skillet until golden. They won't have grill marks but it's still important to heat up and lightly cook the ingredients.