Preheat and prep the pan.
Heat the oven to 375°F (190°C). Line an 8-inch (20-cm) square pan with parchment paper (leave a little overhang for easy lifting), or grease it well.

Grate and squeeze the sweet potato.
Peel and coarsely grate the sweet potato to measure 2 cups (260 g). Wrap the shreds in a clean towel and squeeze lightly over the sink—just enough that it doesn’t feel dripping wet.

Mix the batter.
In a large bowl, whisk the eggs until well combined. Add the squeezed sweet potato, chopped spinach, feta, oat flour, baking powder, oregano, salt, and pepper. Stir until everything is evenly mixed and the spinach is well distributed.

Press into the pan.
Scrape the mixture into the prepared pan. Use a spatula to press it down firmly and smooth the top. Pressing helps the squares hold together and bake evenly.

Bake.
Bake for 28–33 minutes, until the top looks set, the edges are deeply golden, and the center feels firm when gently pressed. If the middle still feels wobbly, give it another 3–5 minutes.

Rest, then slice.
Let the pan rest on a rack for 10 minutes (this is when it finishes setting). Lift out using the parchment overhang, then cut into squares.