Is this too much of a mouthfull?
“Mr. Nuttel, you may wonder why we keep that window open on such a cold October day,” she said, motioning toward a large, French window that opened on to a green, pristinely manicured lawn.
I don't think so. It includes the statement plus the reason for that statement. The only suggestion I have is to correct the spelling to "onto" (one word).
=)