Preheat oven to 400 F.
Cut cauliflower into quarters and remove the core. Place into a 2 1/2 qt. baking dish. Drizzle with 1 1/2 tablespoons olive oil, 1/2 teaspoon salt and 1/4 teaspoon pepper. Bake 30 to 40 minutes until browning and tender. Remove from the oven and let cool slightly.
While cauliflower is cooking, bring a pot of water to boil with 1 1/2 teaspoons salt. Blanch the green beans until barely tender, about 4 minutes, then drain and add quickly to the ice water.
Break the cauliflower into florets. In a blender, puree with chicken stock until very smooth, adding more stock if necessary.
Heat a large saute pan over medium-high heat. Saute the mushrooms in remaining olive oil until browned and cooked through. Reduce heat to medium and add butter, shallots, garlic and thyme. Stir and cook until shallots are translucent.
Add cream to the pan and cook, stirring continuously, 4 to 5 minutes, or until reduced by a third. Add cauliflower puree to the pan, along with remaining salt and pepper. Stir and cook until heated through. Remove from the heat and stir in lemon zest and Worcestershire sauce.
Drain the green beans well and add to the baking dish. Pour in the cauliflower cream and stir gently to combine. Spread evenly and top with Parmesan. Bake until brown and bubbling, 10 to 15 minutes.