Preheat oven or toaster oven to 350°F. Arrange buns, cut sides up, on baking sheet. Top with grated Beemster. Bake 5 minutes or just until cheese melts.

Meanwhile, melt butter in large nonstick skillet. Fry eggs over medium heat to desired doneness.

Arrange tomato, spinach and prosciutto evenly on bottom of buns. Top with eggs and bun tops.