Preheat oven to 375º. Put millet, egg, oil, onion powder, allspice, sage, and salt in a medium bowl or the bowl of an upright mixer. Mix on medium speed for 1 minute (or mix by hand). Add ground turkey. Mix on low speed just until combined.

Squeeze excess water from spinach; then crumble over other ingredients. Mix briefly just to combine. Shape into four patties, using wet hands (mixture will be very soft).

Lightly spray a baking sheet with cooking oil. Arrange burgers on baking sheet. Bake for 25–30 minutes, or until cooked through, turning once. Serve on toasted rolls with mustard and lettuce.