Rinse the quinoa under cold water using a fine-mesh strainer.
In a small saucepan, combine the quinoa and water. Bring to a boil, then reduce heat to low, cover, and simmer for 15 minutes or until water is absorbed. Fluff with a fork and set aside.
Season the chicken breast evenly with half of the sea salt, black pepper, and dried oregano.
Heat half of the olive oil in a skillet over medium-high heat. Add the chicken and cook for 6-7 minutes per side, or until the internal temperature reaches 165°F. Remove from heat and let rest for 5 minutes before slicing into strips.
While the chicken rests, halve the cherry tomatoes and dice the cucumber.
In a small bowl, whisk together the remaining olive oil, lemon juice, and the remaining salt, pepper, and oregano to create the dressing.
Assemble the bowl by placing the baby spinach at the base, followed by the cooked quinoa, sliced chicken, tomatoes, and cucumber.
Drizzle the lemon-herb dressing over the top and serve immediately.