Not sure if this counts as your definition of easy but you can override the theme's post_meta.html partial (i.e. the template used to generate the metadata line after the page title / description) to include this information, here's how I do it on my website. You can then set the lastmod key in the page frontmatter. When both date and lastmod are defined it'll something like this (well the colour change I applied here is separate)

To use .Lastmod parameter, you also need to set enableGitInfo: true in your site’s configuration file.