Heat the oil in a large skillet over medium-high heat. Add the onions and red, green and yellow peppers; cook and stir for about 5 minutes. Add the garlic, and cook for about 2 more minutes. Stir in the quinoa, vegetable stock, and tomato puree.
Return to a boil, then cover and simmer over low heat for 20 minutes, or until quinoa grains are soft. Stir in the diced tomatoes and season with Italian seasoning. Cook until heated through, then serve.