Preheat your oven to 375°F (190°C) and line a standard 12-cup muffin tin with parchment liners or thoroughly grease each cup.
In a large skillet over medium heat, brown the breakfast sausage until fully cooked (about 8-10 minutes), breaking it into small crumbles as it cooks. Drain excess fat and set aside to cool slightly.
Core and dice your Granny Smith apples into small, ¼-inch pieces.
In a large bowl, whisk together the whole wheat flour, rolled oats, baking powder, baking soda, salt, dried sage, dried thyme, and black pepper.
In a separate bowl, whisk together the maple syrup, eggs, applesauce, olive oil, and Greek yogurt until smooth.
Gradually add the wet mixture to the dry ingredients, stirring until just combined. Do not overmix.
Fold in the cooled sausage, diced apples, shredded cheese, and walnuts (if using).
Divide the batter evenly among the prepared muffin cups, filling each about ¾ full.
Bake for 25-30 minutes, or until a toothpick inserted into the center comes out clean and tops are golden brown.
Allow to cool in the pan for 5 minutes before transferring to a wire rack to cool completely.