Overriding virtual methods when instantiating a class

I have a class with some virtual functions, let's pretend this is one of them:

public class AClassWhatever
{
    protected virtual string DoAThingToAString(string inputString)
    {
        return inputString + "blah";
    } 
}

I want to instantiate this class while overriding "DoAThingToAString" inline, much like I can declare properties inline in a declaration, as follows:

...

AClassWhatever instance = new AClassWhatever
{
    DoAThingToAString = new function(string inputString)
    {
        return inputString + inputString + " fill my eyes."
    }
}

...

And now for "instance", DoAThingToAString is overridden with that definition. I need to be able to define the default behavior in the class definition and only override it as needed, differently different times, and I don't want to proliferate a bunch of subclasses to do this.

I know I need to use delegates or something similar, but I've been out of the syntax game for way too long and google was giving me too much irrelevant info.