Rinse 1 cup of uncooked quinoa under cold water in a fine-mesh sieve.
Combine rinsed quinoa with 2 cups of water in a medium saucepan, bring to a boil over medium-high heat, then reduce heat to low, cover, and simmer for 15 minutes.
Remove the saucepan from heat, let quinoa sit covered for 5 minutes, then fluff with a fork and set aside.
Pat 1 pound of boneless, skinless chicken breasts dry and season both sides with ½ teaspoon salt and ¼ teaspoon black pepper.
Heat 1 tablespoon of olive oil in a large skillet over medium-high heat, add the chicken breasts, and cook 6–8 minutes per side until golden and an instant-read thermometer reads 165°F.
Transfer chicken to a cutting board, let rest 5 minutes, then dice into bite-sized pieces.
Dice 1 large cucumber, halve 1 pint of cherry tomatoes, and dice 1 red bell pepper.
In a small bowl or jar, whisk ¼ cup olive oil, 2 tablespoons fresh lemon juice, 1 teaspoon dried oregano, ½ teaspoon Dijon mustard, ¼ teaspoon salt, and a pinch of black pepper until emulsified.
Divide 5 ounces of mixed greens among four bowls. Top each with cooked quinoa, diced chicken, cucumber, cherry tomatoes, red bell pepper, and diced ripe avocado.
Drizzle each bowl with the lemon herb vinaigrette just before serving.