Wash hands with soap and water.
In small bowl, stir together miso, mayonnaise, and 2 tbsp vinegar. Place ribs on a rimmed baking sheet and season lightly with salt. Do not rinse raw meat. Pour all but 2 tablespoons miso mixture over ribs; turn to coat. Set remaining miso mixture aside.
Wash hands with soap and water after handling uncooked meat.
Thinly slice radishes and toss in a medium bowl with remaining 1 tbsp vinegar. Season with salt and toss again.
Heat a large heavy skillet, preferably cast iron, over medium high. When pan is hot, add 2 tsp. oil and swirl to coat. Remove ribs from marinade. Do not reuse marinade used on raw meat.
Wash hands with soap and water after handling uncooked meat.
Add half of ribs to pan and cook without turning until underside is very dark brown and charred in spots, about 2 minutes. Turn ribs and cook until second side is golden brown, 1–2 minutes, and internal temperature reaches 145°F on food thermometer. Transfer to a plate. Repeat with remaining 2 tsp. oil and remaining ribs. Let rest 5 minutes.
Season reserved miso mixture with Sriracha; adjust to taste. Add additional vinegar, if desired. Cut ribs crosswise into 2 or 3 pieces; serve over bowls of rice, topped with watercress and drained radishes and drizzled with spicy sauce.