Preheat the oven to 400°F (200°C).

Place half of the broccoli florets in a steamer basket; place the basket in a large saucepan over 1-inch (2.5 cm) of water. Bring to a boil, cover, and steam for 3-4 minutes until crisp-tender. Transfer to a greased 13x9-inch (33x23 cm) baking dish. Repeat with the remaining broccoli.

In a small saucepan over medium heat, melt 4 tablespoons (55 g) of butter. Add the finely chopped onion and sauté until tender, about 4-5 minutes.

Stir in the minced garlic and cook for an additional minute.

Add the flour, stirring until blended. Gradually add the milk, bringing the mixture to a boil. Cook and stir for 2 minutes or until thickened.

Ladle a small amount of the hot mixture into the beaten egg yolk, stirring quickly to temper the yolk. Return all to the pan, stirring constantly, and cook for another minute.

Remove the saucepan from heat and stir in the grated Parmesan cheese, salt, and pepper until smooth.

Pour the cheese sauce evenly over the broccoli in the baking dish.

In a small skillet, melt the remaining 2 tablespoons (30 g) of butter and add the seasoned bread crumbs. Cook until golden brown, about 3-4 minutes.

Sprinkle the browned breadcrumbs over the top of the casserole.

Bake uncovered for 15-18 minutes, or until heated through and bubbly.