Start by slicing the fresh king oyster mushrooms into ¼-inch thick pieces. If extra-large mushrooms, halve them for better marinade absorption. Lay the sliced mushrooms evenly sized for uniform cooking.

In a medium bowl, whisk together soy sauce, gochujang, mirin, sugar, minced garlic, and sesame oil until well-combined and smooth.

Gently coat the sliced mushrooms in the marinade, ensuring each piece is covered. Allow marinating for at least 20 minutes at room temperature, or refrigerate overnight.

Heat a cast-iron pan over medium heat and add a splash of neutral oil. Once shimmering, add a batch of the marinated mushrooms and sauté for 2-3 minutes on each side.

Return all mushrooms to the pan, pour in remaining marinade, increase heat, and cook for about 3-4 minutes, stirring occasionally until the sauce thickens.

Transfer the hot mushrooms to a serving dish paired with steamed rice, garnishing with cucumber, sesame seeds, and scallions. Serve warm.