In a large bowl, add the ricotta, crumbled feta, grated parmesan, sliced onion, eggs, olive oil, milk, and spinach.
Mix the wet ingredients together using your hands.
Add the SR flour to the bowl. If you're using plain, buckwheat, brown rice, or a combination of those flours, add the baking powder as well.
Gently mix everything together by hand until just combined. Do not overmix.
Spray a muffin tin well with cooking spray.
Wet your hands, form the batter into large balls, and place them into the muffin tin.
Preheat oven to 220°C (420°F).
Place muffins in the oven, then immediately reduce temperature to 180°C (350°F).
Bake for 20–30 minutes, or until golden, springy to the touch, and a skewer inserted into the center comes out clean.
Cool slightly, remove from muffin tin and place on a wire rack before serving.