The energy sector is facing one of the major challenges of recent decades: transiting from a fossil-based energy system to a renewable-based system. Significant operational flexibility will be required to cope with the variability and uncertainty inherent to variable renewable power. Flexibility options such as transmission grid enhancements, energy storage systems, and demand response resources are crucial for achieving a robust and efficient 100 % renewable energy system. How should the optimal mix of flexibility options be designed? To answer this question, we have developed CREST, a tool for optimizing the transition to a 100 % renewable energy system for Europe and beyond. CREST innovative solution method leverages high-performance distributed computing in energy system planning under uncertainty. CREST reduces solution times by more than 90 % and enables optimizing large-scale energy systems that cannot be handled by existing methods.