After my unit tests showed all green, there was still a bug in my code.

System.NotSupportedException: LINQ to Entities does not recognize the method ‘System.Collections.Generic.IList`1[iCal.ISchedulerEvent] ProcessItem(iCal.ISchedulerEvent)’ method, and this method cannot be translated into a store expression.

Solution is to call the ToList() method to get the data into dotNET.

var possibleResults = DbSet
        .Where(item => item.Start < request.End)
        .ToList(); // else the link-to-entities will complain
var result = possibleResults
        .SelectMany(item => ProcessItem(item))
        .OrderBy(item => item.Start)
        .FirstOrDefault();