Grate the zucchini and weigh it, transfer to a colander and sprinkle 1/2 tsp salt on top. Let sit for 5 minutes while preparing the other ingredients. Add the grated onion to drain as well. It does not need salt.
Preheat your oven to 400°F and line a loaf pan with parchment paper
In a large bowl, whisk together almond flour, potato starch, millet flour, baking powder, salt, and black pepper.
Sqeeze out the excess moisture from the zucchini and onion with your hands, or the back of a spoon in the colander. It doesn't have to be completely dry.
Add the zucchini, and onion to the flour blend and toss to coat. Mix in the grated cheese.
In a separate bowl, whisk milk, and eggs, then sprinkle the psyllium husk powder on top. Immediately whisk the psyllium to prevent lumps. Whisk in the sourdough discard or yogurt ... if using.
Pour the wet ingredients into the dry and mix until just combined.
Transfer the batter to the prepared loaf pan. Smooth the top.
Sprinkle with a little extra cheese if desired.
Bake at 400 ℉ for 10 minutes, then reduce heat to 350℉ and bake 45-50 minutes more, or until golden brown and a toothpick comes out clean.
Let cool in the pan for 10 minutes before transferring to a rack.
Store covered at room temperature for a few days and then slice and freeze.