Preheat the Oven: Set your oven to 375°F (190°C). Grease a standard 12-well muffin tin generously with cooking spray or butter.

Cook the Sausage and Spinach: In a medium frying pan, heat the vegetable oil over medium-high heat. Add the breakfast sausage, breaking it into small pieces, and cook until browned and cooked through, approximately 5 minutes. Add the baby spinach and cook until wilted, about 2 minutes. Remove from heat and allow the mixture to cool to room temperature, roughly 15 minutes.

Prepare the Dry Ingredients: In a large bowl, whisk together the all-purpose flour, baking powder, baking soda, salt, and freshly ground black pepper until well combined.

Combine Wet Ingredients: Melt the unsalted butter and let it cool slightly. In a separate medium bowl, whisk together the buttermilk, eggs, and melted butter until smooth.

Mix Wet and Dry Ingredients: Pour the wet mixture into the bowl with the dry ingredients. Stir gently until just combined; the batter will be slightly lumpy, which is fine. Avoid overmixing to ensure tender muffins.

Incorporate Fillings: Fold the cooled sausage and spinach mixture, shredded sharp cheddar cheese, finely diced apple, and chopped fresh chives into the batter until evenly distributed.

Fill the Muffin Tin: Spoon the batter into the prepared muffin tin, filling each well about three-quarters full to allow room for rising.

Bake: Place the muffin tin in the preheated oven and bake for 20-25 minutes, or until the muffins are golden brown and a toothpick inserted into the center comes out clean.

Cool and Serve: Once baked, remove the muffins from the oven and let them cool in the tin for 5 minutes. Then, transfer them to a wire rack to cool slightly before serving warm.