Autonomous vehicles (AVs) are rapidly evolving as an innovative mode of transportation. However, the consensus in both industry and academia is that AVs cannot independently resolve all traffic scenarios. Consequently, the need for remote human assistance becomes clear. To enable the widespread integration of AVs on public roadways, it is imperative to develop novel models for remote operation. One such model is tele-assistance, which promotes delegating low-level maneuvers to automation through high-level directives. Our study investigates the design and evaluation of a new command-based tele-assistance user interface for the teleoperation of AVs. First, by integrating various control paradigms and interaction concepts, we created a simulation-based, high-fidelity interactive prototype consisting of 175 screens. Next, we conducted a comprehensive usability study with 14 expert teleoperators to assess the acceptance and usability of the system. Finally, we formulated high-level insights and guidelines for designing command-based user interfaces for the remote operation of AVs.