Step 1

Stir miso, mayonnaise, and 2 Tbsp. vinegar in a small bowl. Place ribs on a rimmed baking sheet, pat dry, and season lightly with salt. Pour all but 2 Tbsp. miso mixture over ribs; turn to coat. Set remaining miso mixture aside.

Step 2

Thinly slice radishes and toss in a medium bowl with remaining 1 Tbsp. vinegar. Season with salt and toss again.

Step 3

Heat a large heavy skillet, preferably cast iron, over medium-high. When pan is hot, add 2 tsp. oil and swirl to coat. Cook half of ribs 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. Transfer to a plate. Repeat with remaining 2 tsp. oil and remaining ribs. Let rest 5 minutes.

Step 4

Season reserved miso mixture with Sriracha, taste it, and adjust until it’s the right amount of spicy for you. Add more 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.