elixir phoenix - put_flash/3 in the plug

I created a plug to check if the user already created a profile, and redirect them to the /profiles/new page with a flash if they not yet have one:

Plug.CheckProfile

case profiles do
  nil ->
    conn |> put_flash(:info, "No profile found.") |> redirect(to: "/profiles/new") |> halt()
  profiles ->
    assign(conn, :profiles, profiles)
end

but I read somewhere that put_flash is normally for the controller, what is the proper way of doing this?

Is there an alternative for plug?

A controller is just another plug. This isn't a super-common use case -- you're allowed some leeway in how to implement the behavior. A plug/middleware seems like a clean solution. At its heart, it isn't all that different from the conditional redirects you see that vary depending on whether a user is logged in or not.