Using a partial render across all pages in a app

I have written a test app with a Controller called pages. Pages has index, about, contact, and doc in the Pages controller. I want to create a widget that is a partial render that will be included in all of them with the default layout. The widget will display data from a model. I got this to work by creating a variable in each method in the Pages controller that are all the same thing. Howdo I do this without duplication all that effort?

For example in my Pages controller I have:

class PagesController < ApplicationController
  
  def index
    @blah="42"
  end

  def contact
     @blah="42"
  end

  def about
     @blah="42"
  end

  def doc
     @blah="42"
  end
end

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I want to provide the data to the partial without having to repeat it everywhere.