Place the millet and the quinoa in a fine sieve. Rinse the grains under water for a minute and shake them dry.
Add the grains to a pot, along with the water, maple syrup, and salt. Bring the mixture to a boil and lower it to a simmer. Simmer for 15 minutes, or until all of the water has been absorbed. Add the lemon zest and juice to the pot, fluff the grains lightly with a fork, cover, and allow them to rest for 15 minutes.
While the grains cook and rest, toast your cashews in a small skillet over low heat until they're lightly golden (this step isn't necessary if you have roasted cashews already).
To prepare the dish, simply mix the blueberries and cashews into the quinoa. Divide into bowls and serve, topped with a little extra lemon zest for garnish, if desired.