Tamago-don is a homely thing. It's the stuff of busy, frugal mothers trying to throw something together for lunch on a Sunday afternoon. It's the kind of thing a salaryman trying to save a couple hundred yen on lunch might choose during a particularly rough month. It's comfort food.
Little more than sautéed onions with eggs, seasoned with a heavily mirin-sweetened, soy-sauce based dashi base, cooked to a soft curd, the prospect of tamago-don won't trigger a lot of enthusiasm in most Japanese, but perhaps you'll catch a hint of wistful nostalgia.
On the other hand, if you offered to cook such a thing when the weather is bad and spirits are low, your efforts would probably be appreciated.
And if, for example, you happened to have a huge supply of local morels that you really needed to make use of, and some just-picked mizuna greens, and perhaps a little chopped negi, you might be able to enliven this dish just enough to make it interesting.
If you happened to use a heavy hand with said morels, and sauteed them until almost, but not quite charred, along with the softened onions, then added a little seasoned dashi, folded this into an omelet pan full of your seasoned egg mixture, stirred the curds, and poured the just-barely-set eggs over rice, you might have something else entirely.
You might consider how much the shape of morels, more properly translated as amigasatake, resembles the tops of tsukushinbo, or horsetail shoots.
In that case, you might call this variation of tamago-don tsukushinbo-don.
Up way too close
Well, it's still tamago-don. It's not life a life-altering transformation, but it's certainly a worthy use of an excess of morels. I'll probably continue to cook my morels with lots of butter or olive oil, but when I'm looking for a more assari taste, this is a good alternative.
Note: Hiromi deserves all the credit for seeing the similarity between tsukushinbo and morels.