Cooked frozen eggs

So we welcomed our third child last week and some folks have brought by some casseroles and dinners for us. There’s a local place that does casseroles you can buy frozen and just heat or cook when you’re ready. One of them had eggs, browned sausage chunks, cheese, and a little bit of biscuit. I have no idea how they do it, so I’m asking the FDB. How did they cook eggs, then put them in a casserole, and have them light, buttery, and fluffy when I put them in the oven at 350 for about 45-55 minutes? I’d love to make some stuff like this and toss in my deep freeze but the thought of horrible egg keeps me from doing so.