Rinse the quinoa under cold water in a fine-mesh strainer to remove any bitterness.
Combine the rinsed quinoa with 0.5 cup water in a small saucepan; bring to a boil, then cover, reduce heat to low, and simmer for 15 minutes until the water is absorbed.
While the quinoa cooks, slice the chicken breast into bite-sized pieces and season with half of the sea salt, black pepper, and dried oregano.
Heat 0.5 tablespoon of the olive oil in a non-stick skillet over medium-high heat and sauté the chicken until golden brown and cooked through, about 6-8 minutes.
Halve the cherry tomatoes, dice the English cucumber, and finely mince the red onion and garlic.
In a small bowl, whisk together the remaining 0.5 tablespoon of olive oil, lemon juice, minced garlic, and the remaining salt, pepper, and oregano to create the vinaigrette.
Once the quinoa is finished, fluff it with a fork and transfer to a large serving bowl.
Add the sautéed chicken, tomatoes, cucumber, and red onion to the bowl with the quinoa.
Pour the lemon-herb vinaigrette over the ingredients and toss thoroughly to combine.
Garnish with freshly chopped parsley and serve immediately while the chicken is warm.