Render-array.php

<?php

$render_array = [
	// My render array full of items.
]

$render_context = new RenderContext();
$this->renderer->executeInRenderContext($render_context, function() use (&$render_array) {
  $this->renderer->render($render_array);
});

$bubbleable_metadata = $render_context->pop();
$bubbleable_metadata->applyTo($render_array);
$content = $this->renderCache->getCacheableRenderArray($render_array);

$content['#cache']['tags'][] = 'rendered';

$response = new HtmlResponse($content, 200, [
  'Content-Type' => 'text/html; charset=UTF-8',
]);

return $response;