Preheat the oven to 400°F (200°C) and line a baking sheet with parchment paper.

In a medium saucepan, bring the quinoa and water or broth to a boil. Reduce the heat to low, cover, and simmer for about 15-20 minutes, or until the quinoa is tender and the water has been absorbed. Fluff the quinoa with a fork and set it aside.

In a large skillet, heat the olive oil over medium-high heat. Add the chicken and cook for about 5-7 minutes, or until it's cooked through and lightly browned. Remove the chicken from the skillet and set it aside.

In the same skillet, add the diced onion and cook for about 3-4 minutes, or until it's translucent and starting to caramelize. Add the minced garlic and cook for an additional 1-2 minutes, stirring constantly to prevent burning.

Add the mixed vegetables to the skillet and cook for about 2-3 minutes, or until they're tender-crisp. Season with salt and pepper to taste.

In a large bowl, combine the cooked quinoa, chicken, onion mixture, and chopped fresh herbs. In a small bowl, whisk together the lemon juice and Dijon mustard. Pour the dressing over the quinoa mixture and toss to combine.

Cover the bowl with plastic wrap and refrigerate for at least 30 minutes to allow the flavours to meld together.

Serve the salad chilled, garnished with additional fresh herbs if desired.