In a medium sauté pan over medium-high heat, add the red onions and season with kosher salt to taste. Sauté about 2 minutes to soften onion. Add the pineapple and jalapeño and cook until softened, another 2 minutes. Add the rum and flambée. Add the kumquats and turbinado sugar. Then add the soy sauce. Taste and adjust seasoning. Reserve and keep warm.
Make the shrimp: Season the shrimp with salt, then dip in butter, then in the potato flakes, coating both sides well. Over medium heat, add the remaining butter to a large cast iron skillet and add the shrimp. Cook about 3 to 4 minutes on the first side and about 1 ½ minutes on the other, until golden brown and crispy. Transfer shrimp to a paper towel-lined plate.
To serve, place the shrimp on serving plate, drizzle with warm salsa, and garnish with shiso chiffonade and your choice of yuzu, togarashi, or chile powder.