Brussels Sprouts Gratin

About 1.5 pounds of Brussels sprouts, halved
2 Tablespoons butter
2 Tablespoons flour
1.5 cups milk
1 cup parmesan cheese, grated
1/2 teaspoon kosher salt
1/4 cup mayo
black pepper to taste

Pre-heat oven to 350
Clean, trim stems of Brussels sprouts. Cut in half and keep all of loose leaves that fall off. Fill medium stock pot half way with water, bring to boil. Add Brussels sprouts, stirring in hot water and blanch quickly for about 2 minutes or until they turn bright green. Immediately drain. Do not cook Brussels sprouts and make them soft!
In large pan melt butter, then add flour and quickly stir with whisk to combine and remove clumps. Turn off heat.
Add milk, salt, cheese, mayo and black pepper. Stir until cheese is melted.
Add Brussels sprouts and combine with cheese mixture. Pour into baking dish and bake for about 30-45 minutes or until Brussels sprouts are tender.
For extra cheese crust, sprinkle cheese on about 10 minutes before Brussels sprouts are finished.