Module for recommendations based on Solr
Actually, if you go to Apache Solr's 'Pages/blocks' configuration page at admin/config/search/apachesolr/search-pages, you should see whether your "More like this" (which is like a "Related content") block is enabled.

If it's not, just click on the "Add search block "More Like This" as shown below:



That will take you to a page where you can configure your 'More Like This' block. 


you can title it and call it something like "Similar Products", 
you can define which content types it recommends (if you only want articles, or regular pages or just products to appear in this block) --> set that in the 'Filters' section which is collapsed by default
you can configure settings about the word matching it uses to pull up relevant content (how frequently words must appear or excluding words that are too short or too long from being matched on, etc)


Would this default block that comes with Sorl meet your needs?