Using fillings that are too wet

Water is abundant in vegetables. Cucumbers, lettuce, and celery, for example, are composed of more than 90% water. For good reason, you've probably never heard of a cucumber quiche. If your quiche is soggy, it's possible that it's not due to a flaw in the crust or under-baking. Using too much water with the vegetables will result in a soggy quiche.

In your filling, avoid using waterlogged vegetables. Avoiding all water in your filling is nearly impossible with vegetables, but you can take steps to remove as much liquid as possible. Greens, such as spinach, expel a lot of moisture. After cooking, squeeze the spinach to remove any excess water. If you're going to use highly liquid vegetables like tomatoes and zucchini, make sure to pre-cook them first. Raw tomato slices may look lovely, but they will make your quiche appear underbaked.