Bring the quinoa and water to a boil in a saucepan over high heat. Reduce heat to medium-low, cover, and simmer until the quinoa is tender, and the water has been absorbed, about 15 to 20 minutes. Spread into a mixing bowl, and refrigerate until cold.
Once the quinoa has chilled, gently stir in the tomatoes, cucumber, and onion. Season with lime juice, cumin, salt, and pepper; stir to combine. Divide the spinach leaves onto salad plates, and top with the quinoa salad. Garnish with the avocado slices to serve.
