In a small bowl, make the balsamic vinaigrette by whisking balsamic vinegar, olive oil, dijon mustard, garlic, salt and pepper to taste. Set aside.
In a large bowl, add cooked and cooled quinoa, arugula, onion, tomato and balsamic vinaigrette. Season with salt and pepper. Gently toss and set aside.
Generously season salmon filets with salt and pepper. In a skillet, heat olive oil over medium heat. Place salmon on skillet, skin side up, and cook for 4 minutes, undisturbed, until golden brown and crispy. Flip the filets and cook until the fish feels firm to the touch, about another 3-4 minutes. Transfer to a plate to cool.
Once the salmon fillets have cooled, break them apart using a fork and toss in the quinoa salad. Squeeze some lemon juice, if desired. Serve and enjoy!