First, always let your quiche rest for at least 10 minutes after baking. It will finish setting up as it cools, allowing each slice to hold its shape. Be sure to cool the vegetables until all of the moisture evaporates as well. Any extra liquid will affect the overall texture. If the center is still runny or watery, it’s possible that the ratio of dairy to eggs was off. Be sure to use large eggs so the mixture will set properly.