This is a classic example of the muscle cyst form of Sarcocystis. This apicomplexan parasite has a lifecycle similar to Toxoplasma gondii. Depending on the species, humans can be the definitive host (and contract a GI infection) or the intermediate host (and form muscle cysts). This particular species is S. cruzi, which is not a human pathogen. However, S. hominis is also found in beef and can be transmitted to humans. Interestingly, S. hominis has been not been reported in the U.S., but may be found in beef from many other regions, including Europe.
Congratulations to Dr. R - the only one to get this correct (I told you it was tricky!)