Place water in a large saucepan and bring to a boil. Add the soy sauce and quinoa, cover and let simmer over medium heat for about 20 minutes, until all liquid has been absorbed.
In the meantime, heat 1 tablespoon of olive oil in a large skillet over medium-high heat.
Cook and stir red bell peppers and onions until tender, about 10 minutes. Add the mushrooms and tomatoes and cook for another 5 minutes.
Remove from heat and combine the cooked vegetables and the quinoa in a bowl.
Combine all the dressing ingredients and pour over the quinoa. Finely slice the preserved lemon and add to the bowl with the mint.
Taste and adjust the salt and pepper if needed.
Cover and chill for at least 2 hours before serving.
Top with fresh mint leaves if desired.