Preheat oven to 400 degrees F (200 degrees C).
In a medium saucepan, bring Vegetable Broth (1 can) to boiling. Add Quick Cooking Barley (1/3 cup) and Quinoa (1/3 cup). Return to boiling; reduce heat to low. Cook, covered, about 12 minutes or until tender.
Drain, reserving one-third cup cooking liquid; set aside.
In a large skillet, cook Onion (1/2 cup) and Garlic (2 clove) in hot Olive Oil (2 tablespoon) over medium heat for 2 minutes.
Add Mushroom (2 cup), cook and stir for 4-5 minutes or until tender. Stir in Diced Tomatoes (1 can), Fresh Baby Spinach (4 cup), Salt (1/2 teaspoon), and Freshly Ground Black Pepper (1/4 teaspoon).
Add quinoa mixture and 1/2 cup of Pepper Jack Cheese (1 1/4 cup) stir to combine. Remove from heat.
Cut Bell Pepper (3) in half lengthwise. Remove and discard seeds and membranes from the peppers. Sprinkle insides of peppers lightly with additional salt and black pepper.
Fill pepper halves with quinoa mixture. Place peppers, filled sides up, in a three-quart rectangular baking dish. Pour the reserved cooking liquid into dish around peppers.
Bake covered for 30 minutes. Uncover; top stuffed peppers with remaining cheese. Bake uncovered about 10 more minutes or until peppers are crisp-tender and cheese is browned.