1. Heat oil in a large saucepan over medium-low heat. Add onion and cook, stirring, until softened (2-4 minutes). Stir in millet and garlic and cook, stirring, until fragrant (30 seconds).
2. Add water and salt and bring to a boil over medium heat. Reduce heat to low, cover and cook, stirring once or twice, until almost tender (20 minutes).
3. Stir in remaining ingredients and cook, uncovered, maintaining a simmer and stirring often to keep the millet from sticking, until the mixture is thick and the liquid has been absorbed (10 minutes).
4. Remove pan from the heat and let stand until cool enough to handle (30-40 minutes).
5. When mixture is safe to handle, use damp hands to shape the millet mixture into 12 (3") patties.
6. Coat a large nonstick skillet with cooking spray and heat over medium. Working in batches, add millet cakes and cook until the bottoms are browned (3-5 minutes). Carefully turn the cakes with a wide spatula and cook until the other side is browned (3-5 minutes more).