For the sauce, mix all ingredients in a medium bowl and chill until serving.
For the meatloaf, line a rimmed baking sheet with parchment paper and preheat the oven to 325F.
In a large mixing bowl, combine the turkey, spinach, almond flour, eggs, onion, salt, marjoram, and pepper. Mix with your hands until very well combined.
Place half of the turkey mixture on the prepared baking sheet and shape into a flat rectangle, about 1 inch thick. Cover with 3/4 cup of the feta cheese. Place remaining turkey mixture overtop of feta and shape the whole thing into a rough loaf. Sprinkle with the remaining feta, pressing lightly to adhere.
Bake for 1 to 1 1/4 hours, or until the internal temperature reaches 160F on an instant read thermometer.
Cut into slices and drizzle with tzatziki sauce, passing remaining sauce at the table.