You can pipe the %Phoenix.LiveView.Rendered into
Phoenix.HTML.Safe.to_iodata() |> to_string()
which would return the html resulting string.

indeed. Some times things are so obvious

defmodule Sample do
  require EEx

  EEx.function_from_file(
    :def, :sample, "sample.html.heex", [:assigns], engine: Phoenix.LiveView.HTMLEngine)
end

defmodule MyComponent do
  use Phoenix.Component

  def greet(assigns) do
    ~H"""
    <p>Hello, <%= assigns.name %></p>
    """
  end
end

# sample.html.heex
<MyComponent.greet name={@name} />

iex(1)> Sample.sample(%{name: "World"}) |> Phoenix.HTML.Safe.to_iodata() |> to_string()
"<p>Hello, World</p>"

If someone is coming here to do it the hard way (without embed_templates, eg you need more control).
This changed with LV 0.18, one now has to:

EEx.function_from_file(
          :def,
          :fun_name,
          source,
          [:assigns],
          engine: Phoenix.LiveView.Engine,
          caller: __ENV__,
          source: source
)