Preheat the oven to 375°F (190°C). Prepare a muffin tin by greasing each cup or lining with paper cups.

In a large bowl, mix together the flour, flaxseed, baking powder, salt, garlic powder, and red pepper flakes.

In another bowl, whisk the eggs and Greek yogurt until smooth.

Pour the wet mixture into the dry ingredients and stir gently until combined. Do not overmix.

Fold in the spinach, chopped chives, and 1 cup of the cheddar cheese.

Spoon the batter evenly into the muffin cups.

Sprinkle the remaining cheddar cheese on top of each portion.

Bake for 20–22 minutes, or until golden and a toothpick comes out clean.

Let cool slightly on a rack and serve warm.