Preheat oven to 375°F. In large skillet, heat oil over medium heat. Add diced vegetables, garlic and herbs and cook, stirring often, until vegetables begin to soften, 3 to 5 minutes. Add quinoa and olives and cook, stirring, 2 minutes. Add salt and cayenne pepper. Remove from heat.
Partially cook peppers in a steamer 4 to 5 minutes. (Pepper halves can also be steamed in a microwave 2 to 3 minutes in a covered dish with 2 tablespoons water.)
Fill pepper halves with quinoa mixture; place on a lightly greased baking dish. Top with marinara or tomato sauce. Add enough water to cover bottom of dish. Cover loosely with foil and bake until peppers are tender, 20 to 25 minutes .