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