Ohio Administrative Law. Of course, even for an audience of lawyers it need only be five minutes long. There is a very small number of people who will stay awake longer than that.
I could probably BS my way through several areas of American history, science fiction, science fiction in American history, and space exploration.
Wildlife of California. Easy.
The key is to digress.
Scuba diving from beginner’s to technical advanced diving.
The basics of small boat sailing and how to get from point A to point B under sail power.
Genetic patterns of inheritance.
I could go on for a couple hours on the topic of greyhound adoption. 
Andrew Lloyd Webber. I know more about him than he does.
The Iditarod; Innovations and strategies throughout the years.
Running Restaurants: Dives to White Linen.
In depth reasons why Liara T’Soni is actually the real hero and best squadmate in the entire Mass Effect (original) series.
I could do a few reasonably well, I think:
Basic double-entry accounting, in my natural somnolent voice. I just have to talk for five minutes and the audience will sleep through the remaining 40. They’ll wake up feeling refreshed and like I did a great job!
But seriously, I could make up simple examples and T accounts that would fill 45 minutes. With digressions.
That sounds like a recipe for some of the best teaching, and some of the worst.
Yeah. I don’t think I’m particularly good at extemporizing (whether in ordinary conversation or if I have to do a presentation about something), so I wouldn’t follow astorian’s approach; but there are enough things I have talked about before (and prepared for then) that I could easily fill 45 minutes talking about them now.
45 minutes without prep work?
I think I would have to talk about Middle Earth & Tolkien with some references to Mythology, Faerie Tales and the like.
Maybe Yankee baseball. I’m sure I could fill 45 minutes.
I have actually spent 45 minutes teaching 3 new hirers about the software our company used at the time, especially the strange old school green screen ERP system we used. But I’m in a different company now and people rarely need as much help with MS Office products these days.
Oh, I could do 45 minutes of Theodore Roosevelt with ease.
Oh, I can digress. I do have tales of literal lions, tigers, and bears (oh my!)
See, that’s why I think the key to this whole question is the ability to take the topic and then use it as jumping off point to go off on long and winding tangents.
Related tangents, but tangents nonetheless. A few instances of, “Now, where were we? Ahh, yes!..”
If you do that, it’s easy to eat up 45 minutes.
Various Cannabis-Infused Edibles and How to Make Them At Home.
Silvics of select conifer species in the Pacific Northwest
Probably bees, my current obsession.
The trick is getting me to stop.
I’d give a presentation on the National Parks and Monuments of the West. (45 minutes isn’t enough time though)
Arms acquisition by various countries, or the Dallas Cowboys, or politics of some countries in East Asia or the Middle East.
I’ve had to do that on multiple occasions without picking the subjects. Often for much more than 45 minutes. Okay, they were topics that the organisers knew I could be expected to know something about, but still.
Right now? Completely cold? Probably Julius and Ethel Rosenberg would be easiest.