Preheat a grill to medium high. Soak the red onion in a bowl of ice water. Mix the mayonnaise, garlic, lemon juice and a pinch of salt in a small bowl; refrigerate until ready to use. Toss the peppers with the olive oil and a pinch of salt in a large bowl and set aside.
Season the pork with salt, pepper, the paprika and cumin and rub all over. Brush the grill grates with olive oil. Arrange the peppers and pork on the grill and cover. Grill the peppers, turning once or twice, until charred in spots and crisp-tender, 1 to 2 minutes. Remove to a plate. Grill the pork, turning occasionally, until well marked and just cooked through, 12 to 15 minutes; remove to a cutting board to rest, 5 minutes.
Drain the red onion and blot dry. Thinly slice the pork and arrange on the baguette bottoms. Top with some of the garlic mayonnaise, then the red onion, arugula and bread tops. Serve with the shishito peppers; season with salt.