Melt the butter in a large skillet on medium heat. Add the onions and cook, stirring occasionally until tender, about 5 minutes.
Add the garlic and cook for 1 minute more.
Add the shrimp to the pan and let it cook until almost pink.
Push the shrimp to one side of the pan and add the crawfish tails. Continue to cook until all the shrimp are pink and cooked through, about 5 minutes.
Remove the pan from the stove and set aside.
In a large bowl, whisk the eggs until light. Stir in the cream.
Pour in the contents of pan (the shrimp, crawfish, onions and garlic) into the bowl.
Add the salt, pepper and other seasonings if using.
Add the ham and shredded cheese and stir together.
Pour into buttered pie dish and place the dish on a large baking sheet.
Bake until the quiche is puffed and golden brown, about 1 hour.
Notes
I go mild on my spices when I make this crustless quiche but to give the quiche a "spicier" flavor, add 1/2 teaspoon of Creole Seasoning and/or a few dashes of Tabasco Sauce to the egg/milk mixture before baking it. For single serving quiches, divide the mixture between six to eight buttered ramekins and bake as directed.