In large bowl, beat eggs and milk with fork or wire whisk. Set aside.
In 12-inch nonstick skillet, melt butter over medium heat. Add Canadian bacon and spinach; cook and stir 1 minute, just until spinach begins to wilt.
Pour egg mixture over spinach mixture; add cheese. Cook 3 to 5 minutes, stirring occasionally, until eggs are set but slightly moist. Spoon 1/2 cup egg mixture onto each muffin half.