public class RecommenderContentItem
{
    public Guid Id { get; set; }
    public List<Tag> Tags { get; set; } //A tag is an object which only contains an Id and value, not relevant to the question.
    public double AverageRating { get; set; }
}

public static List<Guid> FilterOnContent<T>(List<T> ratedItems,
        List<T> itemsToFilter)
{
    var cnv = new RciConverter();

    var ratedRcis = new List<RecommenderContentItem>();
    var toFilterRcis = new List<RecommenderContentItem>();
    if (typeof(T) == typeof(Restaurant))
    {
        var rated = (IEnumerable<Restaurant>)ratedItems;
        var toFilter = (IEnumerable<Restaurant>)itemsToFilter;

        ratedRcis = cnv.ConvertMany(rated).ToList();
        toFilterRcis = cnv.ConvertMany(toFilter).ToList();
    }
    if (typeof(T) == typeof(AlgorithmRestaurant))
    {
        var rated = (IEnumerable<AlgorithmRestaurant>)ratedItems;
        var toFilter = (IEnumerable<AlgorithmRestaurant>)itemsToFilter;

        ratedRcis = cnv.ConvertMany(rated).ToList();
        toFilterRcis = cnv.ConvertMany(toFilter).ToList();
    }
    if (typeof(T) == typeof(Dish))
    {
        var rated = (IEnumerable<Dish>)ratedItems;
        var toFilter = (IEnumerable<Dish>)itemsToFilter;

        ratedRcis = cnv.ConvertMany(rated).ToList();
        toFilterRcis = cnv.ConvertMany(toFilter).ToList();
    }
    if (!ratedRcis.Any() || !toFilterRcis.Any())
        throw new TypeArgumentException("Invalid type."); //Custom exception written by John Skeet.

    return ContentBasedFilter.Filter(ratedRcis, toFilterRcis).Select(rci => rci.Id).ToList();
}

public class RciConverter : IConverter<Restaurant, RecommenderContentItem>,
    IConverter<AlgorithmRestaurant, RecommenderContentItem>,
    IConverter<Dish, RecommenderContentItem>
{
    //All 6 methods for converting/mapping here...
}

public interface IConverter<in TSource, out TDestination>
    where TSource : class
    where TDestination : class
{
    IEnumerable<TDestination> ConvertMany(IEnumerable<TSource> sourceObjects);
    TDestination Convert(TSource sourceObject);
}