Rinse the quinoa under cold water, then combine with 0.5 cup of water in a small pot, bring to a boil, reduce heat, and simmer covered for 15 minutes.
While quinoa cooks, season the chicken breast evenly on both sides with sea salt, black pepper, and dried oregano.
Heat the olive oil in a medium skillet over medium-high heat and add the minced garlic, sautéing for 30 seconds until fragrant.
Place the chicken in the skillet and sear for 6-7 minutes per side until the exterior is golden brown and the internal temperature reaches 165 degrees Fahrenheit.
Remove the chicken from the heat, let it rest for 3 minutes, then slice it into thin strips.
Dice the cucumber and halve the cherry tomatoes.
In a large bowl, toss the fresh baby spinach, cucumber, and tomatoes with the fresh lemon juice.
Fluff the cooked quinoa with a fork and add it to the bowl, then top with the sliced lemon-herb chicken.