Preheat your oven to 395°F (200°C).
Grease a muffin pan or line it with muffin liners.
Combine cornmeal, almond flour, baking soda, and salt in a large bowl. Mix well.
Whisk together eggs, olive oil, and sparkling water in a separate bowl until smooth.
Pour the wet ingredients into the dry ingredients and stir until just combined.
Gently fold in chopped spinach and crumbled feta cheese.
Spoon the muffin batter into the prepared muffin pan, filling each cup about 3/4 full.
Bake for 20-25 minutes or until the muffins are golden brown and a toothpick inserted into the center comes out clean.
Allow the muffins to cool in the tin for a few minutes before transferring them to a wire rack to cool completely before serving.