Rinse the millet and red lentils thoroughly in a fine-mesh sieve.
Place the millet and lentils in a small pot with 1.5 cups of water. Bring to a boil, then reduce heat to low, cover, and simmer for 15-18 minutes until tender and water is absorbed.
While the grains cook, season the chicken breast with a pinch of salt and pepper. Grill or pan-sear the chicken over medium heat for 6 minutes per side until fully cooked, then dice into bite-sized pieces.
In a small bowl, whisk together the Greek yogurt, lemon juice, minced garlic, ground cumin, sea salt, and black pepper to create the creamy dressing.
Once the millet and lentils are cooked, remove from heat and immediately stir in the baby spinach so it wilts from the residual heat.
Fold the diced chicken and the creamy yogurt dressing into the warm millet and lentil mixture until well combined.
Serve immediately in a bowl, garnished with extra black pepper if desired.