Preheat oven to 400 degrees. Season salmon lightly with salt and pepper. In a small bowl, whisk to combine miso, rice wine vinegar, sesame oil, and ginger. Add butter and whisk to combine. Season to taste with salt and pepper.
Divide shallots and bok choy between two small brown paper lunch bags. Place salmon filets on top. Carefully spoon or brush miso mixture over salmon, allowing excess to drip off and coat vegetables underneath. Fold bags to close tightly and carefully place on a rimmed baking sheet. Transfer to oven and bake until bags are puffed and slightly charred, 8 minutes for medium-rare. Remove from oven and open bags immediately to prevent overcooking by slicing the tops with a knife or scissors. Be careful, as hot steam will escape.