B767 Captain reported a Leading Edge Slat Asymmetry issue after takeoff. The flight crew performed an air turn back and precautionary landing at departure airport.

Narrative

On Day 0; Aircraft X with routing ZZZ [to] ZZZ1 had an air turn back going back to ZZZ due to a Leading Edge Slat Asymmetry issue after takeoff. At XA31; Aircraft X took-off from ZZZ airport using Runway XXL utilizing a FLAPS 15 for takeoff. During the normal takeoff and climb; our aircraft was on schedule to bring the flaps and slats to a clean configuration. After cleaning the aircraft from FLAPS 5 to FLAPS 1; the Leading Edge Slat Asymmetry appeared on the EICAS alert system. We followed the QRH procedures of the Leading Edge Slat Asymmetry. The aircraft configuration was at Flap 1 position with no alleviation to correct the issue of Leading Edge Slat Asymmetry. At around time XA47 after QRH procedures were followed; we requested priority handling and coordinated with Center and ZZZ Approach to hold at position ZZZZZ at 8;000 ft. to work on performance in-flight calculations for landing performance using FLAPS 20 Vref 30 + 30 for landing. At XB00; we advised Company Dispatch of the condition of our aircraft and holding over position ZZZZZ. We advised Company Dispatch we needed a long suitable runway for landing. At XB25; Company Dispatch relayed message to us via ACARS the performance numbers to land in ZZZ. Company Dispatch advised aircraft needed to burn fuel with a remaining fuel of 33;000 lbs to land back in ZZZ. We concur with this number and that will give our landing weight of 323;500 lbs. This landing weight combined with FLAPS 20 [and] Vref 30 + 30 a landing distance of approximately 7;970 ft. of runway usage which is more than sufficient to land on ZZZ on ILS Runway YYR. At XC08; after burning the excess fuel and with a remaining fuel of 33;000 lbs.; First Officer landed at ZZZ at XC16Z on Runway YYR safely and we had the airport fire fighting crews accomplish a visual and thermal inspection of our aircraft exterior as a precautionary. After the visual and thermal inspection was completed; the fire fighting crews assessed the aircraft is normal. Knowing the aircraft is safe and all normal; we taxied the aircraft into the Ramp with an in time of XC26. Prior to our departure from ZZZ; the inbound crew of Aircraft X had made a discrepancy of Leading Edge Slat Asymmetry in the maintenance log book. Perhaps Maintenance need more time to find the lead cause.It is hard for our maintenance team to duplicate failures that are air related on the ground. Perhaps maintenance need more time to find the lead cause.