Heat oven to 425°F. In large skillet, heat butter over medium heat. Cook onions, stirring occasionally, 20 minutes or until very soft and golden.
Meanwhile, prepare piecrust according to package directions. Gently press crust into bottom and up sides of 11-inch tart pan with removable bottom. Combine cheese, flour, thyme, salt, nutmeg and pepper in separate bowl, then sprinkle mixture over crust.
Transfer cooked onions to tart pan; arrange to make an even layer on top of cheese mixture. In small bowl, whisk together eggs and half-and-half; pour evenly over mixture.
Place tart on cookie sheet, and bake at 425°F 10 minutes. Reduce oven temperature to 375°F; bake 30 minutes longer, or until filling puffs and is just set. Cool slightly. Serve warm or at room temperature.