Preheat the oven to 350 degrees F.
Combine the water and millet in a 2-quart saucepan with a tight-fitting lid. Bring to a boil over high heat, then reduce the heat to medium-low, cover the pan, and cook until the millet is tender, about 20 minutes (see note).
While the millet cooks, sauté the onion in a small skillet over medium-high heat until it turns translucent and starts to brown, about 5 minutes. Add water 1 to 2 tablespoons at a time, as needed, to keep the onion from sticking to the pan.
Add the garlic, sun-dried tomatoes, tamari, sage, fennel, and red pepper flakes, and sauté for another minute to toast the seasonings.
Remove the skillet from the heat. Add the flaxseed meal soaked in water, nutritional yeast and cooked millet, season with sea salt, and mix well.
Using a ¼-cup measure or small ice cream scoop, shape the millet mixture into patties and place them on a nonstick or parchment-lined baking sheet. Flatten the patties slightly.
Bake for 15 minutes, turn over the patties, and continue to bake until the patties are firm to the touch and lightly browned, another 10 minutes or so.