1. Preheat Oven and Prepare Muffin Tin: Start by preheating your oven to 375 F (190 C). Line a 12-cup muffin tin with paper liners or grease it thoroughly with butter or non-stick spray. This prevents sticking and makes for easy cleanup.

2. Combine Dry Ingredients: In a large mixing bowl, whisk together the all-purpose flour, granulated sugar, baking powder, baking soda, salt, and black pepper. Ensure there are no lumps and all ingredients are evenly distributed. This step is crucial for even rising.

3. Mix Wet Ingredients: In a separate medium bowl, whisk together the sourdough discard, milk, melted butter, and egg until well combined and smooth. The mixture should be uniform in color and consistency.

4. Combine Wet and Dry Mixtures: Pour the wet ingredient mixture into the dry ingredient mixture. Stir gently with a spatula or wooden spoon until just combined. Be careful not to overmix; a few lumps are perfectly fine and will lead to a more tender muffin. Overmixing can result in tough muffins.

5. Fold in Onion and Cheese: Gently fold in the finely chopped yellow onion and shredded cheddar cheese (if using) until they are evenly distributed throughout the batter. Again, avoid overmixing.

6. Fill Muffin Cups: Divide the batter evenly among the 12 prepared muffin cups, filling each about two-thirds full. You can use an ice cream scoop for easy and consistent portioning.

7. Bake Until Golden: Bake for 20-25 minutes, or until the muffins are golden brown on top and a wooden skewer or toothpick inserted into the center comes out clean. The tops should spring back lightly when touched.

8. Cool and Serve: Remove the muffin tin from the oven and let the muffins cool in the tin for 5 minutes before transferring them to a wire rack to cool completely. This helps prevent them from getting soggy on the bottom.