
This is almost certainly*
Peziza domiciliana, a mushroom characterized by its habit of emerging in indoor habitats. Usually these are bathrooms, damp basements, and the like. This was one of several that I discovered when I opened my pesticide shed after a rainy weekend. The mycelium appears to be living on wet cardboard.
It goes by common names like "common indoor mushroom," "carpet mushroom," and the literal translation of the scientific name, "Domicile cup fungus." It's in the same broad group as morel mushrooms.
*The longer I do this, the less certain I am of my identifications!