Method

• Position rack in centre of oven; preheat to 200°c
• Toss cauliflower with 1 tablespoon olive oil in large bowl, and spread over a large rimmed baking sheet, spacing apart
• Sprinkle with salt and pepper
• Roast 15 minutes; turn florets over
• Continue roasting until tender
• Cool cauliflower, then thinly slice
• Reduce oven temperature to 180°c
• Press pie crust onto bottom and up sides of 9-inch-diameter tart pan with removable bottom
• Line pie crust with foil; fill with pie weights
• Bake crust 20 minutes
• Remove foil and pie weights; bake until crust is golden, about 5 minutes, pressing crust with back of fork if bubbles form
• Cool and keep at room temperature while you prepare the rest of the tart filling
• Heat remaining 1 1/2 tablespoons olive oil in heavy large skillet over medium heat
• Add the leek; sprinkle with salt and pepper
• Cook until leeks are a deep golden colour, stirring occasionally to keep from browning too much
• Cool slightly
• Brush bottom and sides of crust with mustard
• Spread leek in crust
• Arrange cauliflower evenly over the onion
• Set tart on rimmed baking sheet
• Whisk eggs, mascarpone, cream, seasoning and nutmeg together in medium bowl
• Stir in Gruyère
• Pour mixture over filling in tart pan; sprinkle with Parmesan
• Bake until tart is golden and centre is set, about 40 minutes
• Transfer to rack; cool 15 minutes before serving