Preheat oven to 350 degrees F.
To make chili, heat oil in a large pot over medium-high heat. Add onions and cook for 5-6 minutes, stirring occasionally. Add diced bell peppers and cook for an additional 4-5 minutes. Add mushrooms and garlic and cook for 2-3 minutes.
Stir in diced tomatoes, beans, broth and Stubb’s Chili Cooking Sauce and Spice Pack. Reduce heat to low and let chili simmer for 20-30 minutes, or until thickened. Once chili has thickened, stir in 1 ½ cups cooked tri-colored quinoa to form filling. Set aside.
To assemble bell peppers, cut bell peppers in half and scoop out ribs and seeds. Working in batches, place 3-4 peppers in a microwave safe dish with ¼ cup water. Cover dish with plastic wrap and microwave for 5 minutes or until the bell peppers begin to soften.
Transfer partially cooked bell peppers to a 9x13-inch baking dish and fill with chili and quinoa mixture. Top filled peppers with 1-2 tablespoons cheese and bake for 20 minutes or until peppers are tender and cheese is melted. Serve immediately.