its twiggy now

config/install/system.action.generate_a_technical_metadata_derivative.yml

@@ -14,6 +14,6 @@ configuration:
   derivative_term_uri: 'https://projects.iq.harvard.edu/fits'
   mimetype: application/xml
   args: null
-  destination_media_type: file
+  destination_media_type: fits_technical_metadata
   scheme: public
   path: '[date:custom:Y]-[date:custom:m]/[node:nid]-[term:name].xml'

islandora_fits.module

@@ -42,3 +42,17 @@ function islandora_fits_install() {
         ))->save();
     }
 }
+
+/**
+ * Implements hook_theme().
+ */
+function islandora_fits_theme($existing, $type, $theme, $path) {
+    return [
+        'fits' => [
+            'variables' => [
+                'title' => 'FITS data',
+                'data' => [],
+            ],
+        ],
+    ];
+}

src/Plugin/Field/FieldFormatter/FitsFormatter.php

@@ -131,8 +131,12 @@ class FitsFormatter extends FormatterBase {
             }
         }
 
-        $output = $variables['islandora_fits_fieldsets'];
-        return render($output);
+        $renderable =  [
+            '#theme' => 'fits',
+            '#data' => $variables['islandora_fits_fieldsets'],
+            '#title'  => $this->t("Fits Metadata"),
+        ];
+        return \Drupal::service('renderer')->render($renderable);
     }
 
     /**

templates/fits.html.twig

@@ -0,0 +1,4 @@
+<h1>{{ title }}</h1>
+<div>{{ data }}</div>
+
+