Combine rinsed quinoa and water in a medium pot. Bring to a boil, reduce to low, cover, and cook ~15 minutes until water is absorbed and quinoa is fluffy.
Dice cucumber and red bell pepper; halve tomatoes. Add to a large mixing bowl with spinach.
Whisk olive oil, lemon juice, salt, and black pepper until emulsified.
Remove quinoa from heat, rest covered 5 minutes, then fluff with a fork.
Add fluffed quinoa to the bowl with vegetables. Pour over dressing and gently toss to coat. Fold in chopped parsley and mint.
Taste and adjust lemon, salt, and pepper. Divide into 4 bowls; garnish with more herbs and optional feta. Serve at room temperature.
