So, I gave the recipe a bit of a spin for supper, except that I substituted a bit (and by a bit, I mean a lot). I only made 5 egg-potato cups, just for reference.
I didn't have any russet potatoes, and the recipe calls for baking them in the oven and then letting them chill overnight. I think this animated gif sums up my feelings this instruction:
I instead scrubbed two medium yellow potatoes (and one sweet potato, just to see how it would fare in the recipe) and microwaved them until they were cooked. Then I stuck them in the freezer for 5 minutes, until they were cool enough to handle. I probably should have left them in the freezer longer, as I discovered a few rogue pockets of steam while grating them.
The sweet potato was not as firm as the regular potatoes, so it was more half grated-half mashed, but they still made nice potato cups (I got 2 sweet potato cups and 3 yellow potato in total).
I put finely minced red onion, mushrooms, yellow sweet pepper and cream cheese in the potato cup, before cracking the egg in. They only just fit in my standard size muffin tin.
Getting them out once they were done baking was a bit of an adventure. I definitely did not grease the sides of the muffin tin cups enough, and the bottoms were still a little soft. So, definitely check that your muffin tin is very well greased.
In the end though, they turned out to be delicious. They went really well with buns and spinach salad for dinner. Even munchkin gobbled her share right up.