Cut cauliflower in quarters and remove the core. Add to the Le Creuset casserole, drizzle with 1-1/2 Tbsp olive oil, 1/2 tsp salt, and 1/4 tsp pepper. Bake in oven for 30 to 40 minutes until browning and tender. Remove from the oven and let cool slightly.
While cauliflower is cooking, blanch the green beans in salted boiling water until barely tender, about 4 minutes. Drain and add beans quickly to ice water.
Break cauliflower into florets and purée in a blender with chicken stock until very smooth, adding more stock if needed.
Heat a large sauté pan over medium-high heat and sauté the mushrooms in the remaining 1-1/2 Tbsp olive oil until browned and cooked through. Reduce the heat to medium, then add the butter, shallots, garlic, and thyme. Stir and cook until shallots are translucent. Add the cream and cook, stirring for 4 to 5 minutes until reduced by a third. Add cauliflower with 1 tsp salt and 1/4 tsp pepper, then stir and cook until heated through. Remove from the heat and stir in the lemon zest and Worcestershire sauce.
Drain green beans well and mix together with the cauliflower cream in the casserole dish. Spread evenly and top with Parmesan. Bake in the oven until brown and bubbling, 10 to 15 minutes.
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