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The existence of luminous quasars hosting supermassive black holes (SMBHs) within the first billion years of cosmic history challenges our understanding of black hole growth. However comma much of our understanding relies on the assumption that the observed luminosities are intrinsic to the quasars themselves. It has long been predicted that a large fraction of high-redshift quasars should be gravitationally lensed comma but to date only one lensed quasar above z>5 has been confirmed despite considerable observational efforts. We propose to search for this long sought-after population of lensed quasars with HST-ACS comma targeting six objects at zvirgul6 whose rest-frame UV spectra exhibit exceptionally small proximity zones. These quasars are excellent candidates for being gravitationally lensed objects comma because the local ionizing flux implied by their small proximity zones is far below the expectation from their observed UV luminosities. We aim to apply a two-fold observing strategy using imaging both in the near-IR to search for multiple quasar images comma as well as in the optical regime to target foreground lensing galaxies using the intergalactic medium as a natural long-pass filter to avoid the quasar light comma in order to improve the success rate of this lensing search. In case we do not find evidence for lensing comma the quasars. small proximity zones could be explained by very short UV-luminous lifetimes of less than 10 comma 000 years comma requiring either super-Eddington accretion rates or highly obscured black hole growth phases comma in order to explain the concomitant SMBH growth. Thus either outcome of our proposed observations promises new insights into the formation and growth of the first SMBHs.