Combine 1 tbsp soy sauce, 1 tsp sesame oil, and ½ tbsp mirin in a small bowl.
Add 4-6 oz white fish to a microwave-safe bowl or plate and score.
Season the fish with salt and add thinly sliced peeled ginger.
Pour ⅔ of the soy sauce mixture onto the fish and top with 1 scallion, thinly cut into 1" pieces.
Add bok choy to the microwave-safe bowl or plate and pour rest of soy sauce mixture over bok choy.
Top with microwave-safe lid.
Microwave for 2 min, take bok choy out, and microwave the fish for another 2 min.