  The fact that nonlocality implies steering enables one to certify
steerability by using a Bell inequality violation. Such a certification is
device-independent (DI), i.e., one makes no assumption neither on the
underlying state nor on the measurements. However, not all steerable states can
violate a Bell inequality. Here, we systematically construct a collection of
witnesses for steerable resources, defined by assemblages, in a
measurement-device-independent (MDI) scenario. The inputs driving the
measurement are replaced by a set of tomographically complete quantum states,
and neither the detectors nor the underlying state is characterized. We show
that all steerable assemblages can be detected by properly chosen witnesses.
Furthermore, we introduce the first measure of steerability in an MDI scenario
and show that such a measure is a standard one, i.e., a steering monotone, by
proving that it is equivalent to the steering robustness.
