diff --git a/packages/metro/src/Config.js b/packages/metro/src/Config.js
index f2c97a85..fb96e0e4 100644
--- a/packages/metro/src/Config.js
+++ b/packages/metro/src/Config.js
@@ -25,10 +25,19 @@ import type {PostProcessModules} from './DeltaBundler';
 import type {PostProcessModules as PostProcessModulesForBuck} from './ModuleGraph/types.flow.js';
 import type {TransformVariants} from './ModuleGraph/types.flow';
 import type {HasteImpl} from './node-haste/Module';
+import type {IncomingMessage, ServerResponse} from 'http';
+
+type Middleware = (IncomingMessage, ServerResponse, ?(?Error) => void) => void;
 
 export type ConfigT = {
   assetRegistryPath: string,
 
+  /**
+   * Called with the Metro middleware in parameter; can be used to wrap this
+   * middleware inside another one
+   */
+  enhanceMiddleware: Middleware => Middleware,
+
   extraNodeModules: {[id: string]: string},
   /**
    * Specify any additional asset file extensions to be used by the packager.
@@ -148,6 +157,7 @@ export type ConfigT = {
 
 const DEFAULT = ({
   assetRegistryPath: 'missing-asset-registry-path',
+  enhanceMiddleware: middleware => middleware,
   extraNodeModules: {},
   assetTransforms: false,
   getAssetExts: () => [],
diff --git a/packages/metro/src/index.js b/packages/metro/src/index.js
index fd7c103c..02150175 100644
--- a/packages/metro/src/index.js
+++ b/packages/metro/src/index.js
@@ -69,7 +69,7 @@ async function runMetro({
   maxWorkers = 1,
   projectRoots = [],
   watch = false,
-}: PrivateMetroOptions) {
+}: PrivateMetroOptions): Promise<MetroServer> {
   const normalizedConfig = config ? Config.normalize(config) : Config.DEFAULT;
 
   const assetExts = defaults.assetExts.concat(
@@ -136,6 +136,7 @@ type CreateConnectMiddlewareOptions = {|
 exports.createConnectMiddleware = async function(
   options: CreateConnectMiddlewareOptions,
 ) {
+  // $FlowFixMe I don't know why Flow thinks there's an error here... runMetro IS async
   const metroServer = await runMetro({
     config: options.config,
     maxWorkers: options.maxWorkers,
@@ -144,10 +145,15 @@ exports.createConnectMiddleware = async function(
     watch: true,
   });
 
+  const normalizedConfig = options.config
+    ? Config.normalize(options.config)
+    : Config.DEFAULT;
+
   return {
-    middleware(req: IncomingMessage, res: ServerResponse) {
-      return metroServer.processRequest(req, res);
-    },
+    middleware: normalizedConfig.enhanceMiddleware(
+      (req: IncomingMessage, res: ServerResponse, next: ?(?Error) => void) =>
+        metroServer.processRequest(req, res, next),
+    ),
     end() {
       metroServer.end();
     },