Heat the oven. Preheat the oven to 350°. Coat 4 cups of a muffin tin with cooking spray, making sure each cup is thoroughly coated.
Cook the sausage. In a small sauté pan cook the sausage over medium-high heat until crumbled and cooked through, about 7-8 minutes. Remove sausage from pan and place on a plate lined with a paper towel.
Cook the mushrooms. Return the pan to the heat; add olive oil and mushrooms. Sauté mushrooms for 1-2 minutes until slightly tender. Add mushrooms to sausage.
Whisk the eggs. In a small mixing bowl whisk eggs, milk and ground pepper together.
Prepare the muffins. Divide sausage mixture among the four muffin cups. Pour the egg mixture over the sausage and mushrooms. You may have a little left over depending on the exact size of your muffin cups. Divide cheese evenly among the muffin cups, sprinkling over the top of each.
Bake the muffins. Cook for 25 minutes or until muffin is firm to the touch. Remove from oven and let cool slightly before serving.
Notes
Muffins will keep in an airtight container in the refrigerator for a day or two.