Soak the onion in a bowl of cold water, about 10 minutes. Meanwhile, puree 3/4 cup pineapple with the lime juice, peanut butter, fish sauce and jalapeno until smooth. Toss the cucumber and the remaining 1 1/4 cups pineapple in a bowl, then add the pureed pineapple dressing and toss to coat. Season with salt and pepper.
Pat the pork chops dry and season with salt and pepper. Heat a large skillet over high heat, then add 1 tablespoon vegetable oil. Lay 3 chops in the pan and sear until golden on one side, about 3 minutes; flip and finish cooking, 1 to 2 more minutes. Transfer the chops to a plate and loosely cover with foil. Repeat with the remaining chops, adding the remaining 2 tablespoons oil as necessary.
Drain the onion and add to the pineapple relish. Serve with the pork chops.