Preheat oven to 350°F and coat a 12-cup muffin pan with cooking spray or line with paper baking cups.
Cook ground sausage, drain and set aside. In a medium mixing bowl, whisk together kefir, eggs, melted butter, spinach and pesto. Set aside.
In large mixing bowl combine flour and next four ingredients. Gently fold in cheese. Add wet ingredients to the flour mixture and stir just until dry ingredients are moist. Gently fold in cooked ground sausage until well combined.
Spoon batter into muffin tin, filling completely. Bake for approximately 25 minutes or until golden brown. Cool in pan on wire rack for about five minutes, then remove muffins from pan and finish cooling on wire rack. Serve warm or store in airtight container or bag in refrigerator or freezer.