I don't know a great deal of C#, but couldn't you just do:

    T Log<T>(string msg, T result) {
        Log(msg);
        return result;
    }
    
    var result = operation switch {
        "+" => Log("addition", a + b), 
        "-" => Log("subtraction", a - b), 
        "/" => Log("division", a / b), 
        _ => throw new NotSupportedException()
    };

You already have a lambda for calling Log(), no need for a second to call the inner function. This does change the order of evaluation though.

I just mention it for academic interest. To be honest, I find the idea of passing a parameter to Log() (whether it's the value or the function) that your don't actually want logged, so that you can shoehorn two statements into an expression, abhorrent. Just write the two statements out! I think an old-fashioned switch statement is the right tool here.