Preheat oven to 350°F.
Heat the spinach and let it simmer for a few minutes until most of the liquid has evaporated.
Meanwhile, open the can of tuna, drain it well, and flake it with a fork. Chop the sundried tomatoes into small pieces. Crumble the feta cheese. Peel and mince the garlic or use a garlic press.
In a large bowl, combine the flour, baking powder, oregano, salt, and pepper. Whisk the mixture thoroughly.
Whisk the eggs into the dry ingredients. Add the buttermilk, spinach, tuna, and garlic. Mix until a smooth batter forms.
Fold in the feta and sundried tomatoes.
Grease the muffin tin with olive oil. Distribute the batter evenly among the muffin cups, filling each about ⅔ full, as the batter will rise during baking.
Bake the muffins for 20–25 minutes. To check for doneness, insert a toothpick into a muffin; if it comes out clean, they're ready.
Let the muffins rest in the tin for 5 minutes after baking. Then, carefully remove them using a knife. Enjoy warm or cold.
