Steam cauliflower until just tender, about 5 minutes. Set oven temperature to 350°F.
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 crust. Maintain oven temperature.
Heat remaining 1 1/2 tablespoons olive oil in heavy large skillet over medium heat. Add onion; sprinkle with salt and pepper. Cook until onion is light golden brown, stirring occasionally. Cool slightly.
DO AHEAD: Can be made 1 day ahead. Store crust at room temperature. Cover and chill cauliflower and onion separately.
Brush bottom and sides of crust with mustard. Spread onion in crust. Arrange cauliflower evenly over. Set tart on rimmed baking sheet. Whisk eggs and next 4 ingredients in medium bowl. Stir in Gruyère. Pour mixture over filling in tart pan; sprinkle with Parmesan. Bake until tart is golden and center is set, about 40 minutes. Transfer to rack; cool 15 minutes before serving.
Adapted from Bon Appétit, March 2007