01 - Preheat the oven to 375°F and line a 12-cup muffin tin with paper liners or grease thoroughly.
02 - In a large mixing bowl, whisk together the flour, baking powder, baking soda, salt, and black pepper.
03 - In a separate bowl, whisk eggs, milk, vegetable oil (or melted butter), and Greek yogurt until smooth.
04 - Pour the wet mixture into the dry ingredients and fold gently until just combined, avoiding overmixing.
05 - Fold in cheddar, feta, diced bell pepper, chopped spinach, chives, and any optional add-ins until evenly distributed.
06 - Spoon batter evenly into the muffin cups, filling each about three-quarters full.
07 - Bake for 22 to 25 minutes until golden on top and a toothpick inserted into the center comes out clean.
08 - Let muffins cool in the tin for 5 minutes, then transfer to a wire rack to cool completely before serving.