Preparation Cook cauliflower and broccoli in large pot of boiling salted water until crisp-tender, about 5 minutes. Drain, reserving 2/3 cup cooking liquid. Transfer vegetables to large bowl. Cool. Rinse spinach, then toss in large nonstick skillet over medium-high heat until just wilted. Drain and cool. Squeeze spinach dry; finely chop. Melt butter in heavy medium saucepan over medium heat. Add flour and whisk until smooth, about 2 minutes. Gradually whisk in milk and reserved 2/3 cup vegetable cooking liquid. Whisk constantly over medium heat until sauce thickens and boils, about 3 minutes. Stir in spinach and cheese. Using fingers, coarsely crumble cauliflower and broccoli in bowl. Add spinach béchamel sauce; stir to blend. Season with salt and pepper. Butter 1 1/2-quart baking dish. Spread vegetable mixture in prepared dish. (Can be made 6 hours ahead. Cover and chill.) Preheat oven to 350°F. Bake flan until puffed and heated through, about 25 minutes if at room temperature and 35 minutes if chilled. Serve hot.
Preparation Cook cauliflower and broccoli in large pot of boiling salted water until crisp-tender, about 5 minutes. Drain, reserving 2/3 cup cooking liquid. Transfer vegetables to large bowl. Cool. Rinse spinach, then toss in large nonstick skillet over medium-high heat until just wilted. Drain and cool. Squeeze spinach dry; finely chop. Melt butter in heavy medium saucepan over medium heat. Add flour and whisk until smooth, about 2 minutes. Gradually whisk in milk and reserved 2/3 cup vegetable cooking liquid. Whisk constantly over medium heat until sauce thickens and boils, about 3 minutes. Stir in spinach and cheese. Using fingers, coarsely crumble cauliflower and broccoli in bowl. Add spinach béchamel sauce; stir to blend. Season with salt and pepper. Butter 1 1/2-quart baking dish. Spread vegetable mixture in prepared dish. (Can be made 6 hours ahead. Cover and chill.) Preheat oven to 350°F. Bake flan until puffed and heated through, about 25 minutes if at room temperature and 35 minutes if chilled. Serve hot.