Julius Meinl in Chicago serves baked eggs much like the ones I wrote about last week. (I think we may even have the same baking dishes.) They are excellent, especially the spinach, mushroom, and feta version. I haven't tried replicating it yet, but I think it could be done quickly and easily, especially with good-quality frozen spinach.
While the oven is preheating, sauté some minced onions or shallots, add the mushrooms, salt, and pepper, and then just toss in the spinach and heat through. (I'm thinking that frozen spinach would be nice because it tends to give off less liquid, thereby not stewing the mushrooms, plus of course the prep is quicker -- parboiled greens would be nice too.) Perhaps a splash of cream. And then the usual egg-baking process, with a thicker coat of feta and a bit more time under the broiler. What do you think? Anyway, take a look at their menu. You may be inspired to try some of the other variations: