Bring a large pot of generously salted water to a boil. Add the green beans and cook until bright green and crisp-tender, 3 to 4 minutes. Transfer to a paper towel–lined baking sheet with a slotted spoon. Add the cauliflower to the boiling water and cook until just tender, 3 to 4 minutes. Transfer to the baking sheet with the slotted spoon. Pat the vegetables dry.
Preheat the oven to 375˚. Melt 2 tablespoons butter in a medium saucepan over medium heat. Add the panko and cook, stirring, until golden brown, 4 to 5 minutes; season with salt and black pepper. Transfer the panko to a bowl and let cool; reserve the saucepan. Add the gruyere to the panko and toss; set aside.
Wipe out the saucepan and add the remaining 3 tablespoons butter; melt over medium heat. Add the garlic and cook, stirring, until softened, 1 minute. Sprinkle in the flour and cook, stirring, 1 minute. Gradually add the half-and-half, whisking constantly. Bring to a boil, then reduce the heat and simmer, whisking occasionally, until thickened, about 5 minutes. Add the parmesan, mustard powder, cayenne and 2 teaspoons salt and cook, stirring, until the parmesan is melted, about 30 seconds.
Spread one-third of the cream sauce in a 9-by-13-inch baking dish. Top with the green beans and cauliflower, then the rest of the sauce. Sprinkle with the panko-gruyere mixture. Bake until the gruyere is melted and the sauce is bubbling around the edges, 25 to 30 minutes. Top with chives.
Tools You May Need
Photograph by Ralph Smith
Tools You May Need
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