Rinse the quinoa under cold water, then combine it with water or vegetable broth in a medium saucepan. Bring to a boil, reduce heat, cover, and simmer for 15 minutes. Let it sit for 5 minutes after cooking, then fluff with a fork.

While the quinoa is cooking, prepare the vegetables: halve the cherry tomatoes, dice the cucumber and bell pepper, and finely chop the red onion. Slice the avocado.

In a large bowl, combine the cooked quinoa, cherry tomatoes, cucumber, bell pepper, baby spinach, red onion, and feta cheese (if using).

In a small bowl, whisk together olive oil, lemon juice, salt, and pepper. Pour the dressing over the salad and toss gently to combine.

Serve the salad in bowls or on plates, topping each serving with sliced avocado and garnishing with fresh herbs.