  Pressure represents a clean tuning parameter for traversing the complex phase
diagrams of interacting electron systems and as such has proved of key
importance in the study of quantum materials. Application of controlled
uniaxial pressure has recently been shown to more than double the transition
temperature of the unconventional superconductor Sr$_{2}$RuO$_{4}$ for example,
leading to a pronounced peak in $T_\mathrm{c}$ vs. strain whose origin is still
under active debate. Here, we develop a simple and compact method to apply
large uniaxial pressures passively in restricted sample environments, and
utilize this to study the evolution of the electronic structure of
Sr$_{2}$RuO$_{4}$ using angle-resolved photoemission. We directly visualize how
uniaxial stress drives a Lifshitz transition of the $\gamma$-band Fermi
surface, pointing to the key role of strain-tuning its associated van Hove
singularity to the Fermi level in mediating the peak in $T_\mathrm{c}$. Our
measurements provide stringent constraints for theoretical models of the
strain-tuned electronic structure evolution of Sr$_{2}$RuO$_{4}$. More
generally, our novel experimental approach opens the door to future studies of
strain-tuned phase transitions not only using photoemission, but also other
experimental techniques where large pressure cells or piezoelectric-based
devices may be difficult to implement.
