Preheat the oven to 375 degrees F.

Fill a large pot three-quarters full with water and add enough salt to make the water taste like sea water. Bring to a boil over high heat. Add the cauliflower and cook until crisp-tender, about 3 minutes. Using a slotted spoon, transfer the cauliflower to a baking sheet and set aside to cool.

Add the broccoli to the boiling water. Cook until crisp- tender, 1 to 2 minutes. Drain the broccoli in a colander and transfer to the baking sheet with the cauliflower.

Butter a 15 x 10-inch baking dish (or 2 smaller dishes) with 2 teaspoons of the butter. Arrange the cauliflower evenly in the dish, then arrange the broccoli on top.

Melt the remaining 8 tablespoons butter in a heavy medium saucepan over medium heat. Whisk in the flour and cook, whisking, until the roux is a blond color, about 3 minutes. Add 2 teaspoons salt and the cayenne. Gradually whisk in the milk and whisk constantly until the mixture boils and thickens, about 4 minutes. Whisk in the cheese until it melts.

Pour the cheese sauce over the cauliflower and broccoli, and gently tap the dish on the countertop to remove any air bubbles.

Combine the bread crumbs, Essence, and olive oil in a small bowl, mixing well. Spoon the mixture evenly over the top of the vegetables.

Bake until crumbs are crisp and golden brown, 35 to 45 minutes. Serve hot.