2 oz blue or Gorgonzola cheese, crumbled and divided
2 shallots, diced
3 cloves garlic, minced
2 tbsp Dijon mustard
2 scallions, chopped
Preheat oven to 375°F. Place chicken on a large, rimmed foil-lined baking sheet. Season chicken with salt and pepper. Cover with foil and bake until juices run clear and no longer pink inside, about 25 minutes. Shred or chop into bite-size pieces; place in a large bowl. Reduce oven temperature to 350°F.
Meanwhile, whisk together 1/4 cup vinegar, oil and 1 tbsp chives. Generously brush mixture on all sides of portobellos. Place portobellos in 2 large baking dishes, cap sides down. Pour remaining mixture over portobellos and set aside to marinate for 20 minutes.
Meanwhile, cook quinoa according to package directions.
To bowl with chicken, add quinoa, spinach, remaining 3 tbsp chives, walnuts, 1 1/2 oz cheese, shallots, garlic and mustard. Drizzle remaining 1/4 cup vinegar over mixture; toss to combine.
With portobellos still in baking dishes, fill each with about 1 cup chicken mixture, packing it tightly with the back of a spoon. Sprinkle remaining 1/2 oz cheese over filling. Cover with foil and bake for 15 to 20 minutes.