my_class.rb

# this is running in Project 2
class MyClass
  def some_process
    # if the worker was in this project I could do this
    # MyWorker.perform_async(...args)
    
    # but the worker isn't in my project
    # I always thought I could do this, but it seems it needs
    #   class: ActualClass,
    # not
    #   class: 'ClassName'
    Sidekiq::Client.push({
      class: 'BuildStepWorker',
      args: [ build.id, key ]
    })
    # see https://github.com/mperham/sidekiq/blob/776e69d631092e0c6ffe3130764634666aa062e5/lib/sidekiq/client.rb
    
    # Alternatively I'll just jump in and use RabbitMQ for background tasks
  end
end

my_worker.rb

# this is running in Project 1
class MyWorker
  include Sidekiq::Worker
  def perform(name, count)
    # do something
  end
end