Step 1

Preheat the oven to 400 degrees F.

Step 2

Heat a large braiser or ovenproof skillet over medium-high heat and add the oil. When the oil is hot, add the Brussels sprouts and cook, tossing occasionally, until they begin to brown on the edges, 4 to 5 minutes. Reduce the heat to medium-low and add the butter. When the butter is melted, add the leeks and sprinkle with salt and pepper. Cook, stirring occasionally, until wilted, 4 to 5 minutes. Add the thyme and garlic and cook until fragrant, about 1 minute. Add the flour and cook, tossing until the vegetables are well coated in the flour, about 2 minutes. Gradually stir in the stock and cream and bring to a simmer. Simmer until the sprouts are tender, 2 to 4 minutes. Stir in the Parmesan, mustard and lemon zest. Season with salt and pepper to taste.

Step 3

Top with the crispy onions and bake until the onions are browned and the casserole is bubbly, about 20 minutes.