C# Singleton pattern with triggerable initialization

I need a singleton that:

• is lazy loaded
• is thread safe
• loads some values at construction
• those values can be queried at any time
• the initialization MAY happen at some precise time, before the querying begins - so I must be able to trigger it from the outside somehow. Of course, triggering multiple times should only do the initialization once.

I use .NET 3.5.

I've started with Jon Skeet's implementation (5th version) using a static subclass:

public sealed class Singleton
{
    IEnumerable<string> Values {get; private set;}
    private Singleton()
    {
        Values = new[]{"quick", "brown", "fox"};
    }

    public static Singleton Instance { get { return Nested.instance; } }

    private class Nested
    {
        // Explicit static constructor to tell C# compiler
        // not to mark type as beforefieldinit
        static Nested()
        {
        }

        internal static readonly Singleton instance = new Singleton();
    }
} 

This ticks almost all the boxes, except the "trigger initialization from outside". Since the actual initialization happens inside the ctor, it can't happen more than once.

How can this be accomplished?

The singleton will be used like this:

public static void Main(){

    //do stuff, singleton should not yet be initialized.

    //the time comes to initialize the singleton, e.g. a database connection is available
    //this may be called 0 or more times, possibly on different threads

    Singleton.Initialize();
    Singleton.Initialize();
    Singleton.Initialize();

    //actual call to get retrieved values, should work
    var retrieveVals = Singleton.Instance.Values;

}