Setting it in config.exs would be fine.

For example:

config :demo, DemoWeb.Endpoint,
  render_errors: [
    accepts: ~w(html json),
    root_layout: {DemoWeb.LayoutView, :error},
    layout: {DemoWeb.LayoutView, :skip},
    view: DemoWeb.ErrorView,
  ],
  # ...

{DemoWeb.LayoutView, :error} corresponds to lib/demo_web/templates/layout/error.html.heex.

{DemoWeb.LayoutView, :skip} corresponds to an almost empty template (lib/demo_web/templates/layout/skip.html.heex):

<%= @inner_content %>

You may find that I’m using {DemoWeb.LayoutView, :error}(this is a customized root layout for error pages) instead of the default root layout {DemoWeb.LayoutView, :root}. The reason is from here:

It is worth noting that we did not render our 404.html.heex template through our application layout, even though we want our error page to have the look and feel of the rest of our site. This is to avoid circular errors. For example, what happens if our application failed due to an error in the layout? Attempting to render the layout again will just trigger another error. So ideally we want to minimize the amount of dependencies and logic in our error templates, sharing only what is necessary.

Thank you. That’s what i was looking for. Hard to find

For history, solution looks like this:

defmodule AppWeb.ErrorView do
  use AppWeb, :view

  def render("404.html", assigns) do
    Phoenix.View.render_layout AppWeb.LayoutView, "app.html", assigns do
      render("not_found.html", assigns)
    end
  end
...

Some hacking around get_flash is required though

config :demo, DemoWeb.Endpoint, 
  render_errors: [
    view: DemoWeb.ErrorView, 
    accepts: ~w(json),
    root_layout: false,
    layout: false
  ]

Phoenix.Endpoint has an option called render_errors, you can set root_layout or layout in config.