Mix the breadcrumbs and butter together. Set aside.
In a large pot boil the cauliflower florets for about 2 minutes at a low boil, then add the broccoli florets and boil for 2 more minutes.
Remove from heat and drain well (you want to remove as much excess water as possible). I like to let the veggies sit in a strainer in the sink while I make the cream sauce.
In a large skillet melt 2 tablespoons of butter, then stir in 2 tablespoons of flour. Mix well and cook for about 2 minutes.
Then slowly whisk in the milk, and simmer the mixture until it becomes thick and bubbly.
Stir in the Boursin cheese block until melted. This mixture should be smooth and creamy.
Place the cooked cauliflower and broccoli in an oven safe dish (about 9×13 in size).
Pour the cheese mixture over the veggies. Mix well.
Sprinkle the bread crumb mixture evenly over the top of the casserole.
Bake uncovered for 25 minutes (it should look hot and bubbly).