Can I upgrade to El Capitan in a post MacOS Sierra world?

I Have a 3,1 (late 2009) mac mini that I had used as my HTPC, and I never bothered upgrading past 10.7.5. with the advent of the new Apple TV's it now just collects dust, so I'm pulling it out for my kids to use because our old kid Mac (2006 iMac) is getting really long in the tooth.

I remember prompts (which I unfortunately ignored) to upgrade to El Capitan, but now when I search the AppStore all I see available to me is Sierra. I understand my little old 2.26 Core 2 wont run Sierra. How do I upgrade to a more modern OS?

Thanks.