Separate broccoli into small florets. Cut king oyster mushrooms in half lengthwise and tear them by hand. Slice the onion thinly.

Place broccoli and 2 tbsp water in a microwave-safe container. Cover loosely with plastic wrap and microwave at 600W for 2 minutes. Drain the water.

Season scallops with a pinch of salt and pepper, then lightly coat them with all-purpose flour.

Heat 1 tbsp butter in a frying pan over medium heat. Add scallops and cook until both sides are golden brown. Add the remaining butter, sliced onion, and sauté until softened. Add king oyster mushrooms and cook until tender.

Sprinkle 1 tbsp flour over the mixture and stir until no dry flour remains. Gradually add milk while stirring, and heat over medium heat.