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.