const schema = {
   response: {
      // Augment the current mapping (status code -> response schema) to support
      // status code -> accept header -> response schema
      200: {
         "application/vnd.bla.v1+json": ...
         "*/*": ...
      }
   }
}

fastify.get("/apples/:id", { schema } , (request, reply) => {
  switch (request.header["accept"]) {
    case "application/json":
       reply.send(...);
       break;
   
    case "application/vnd.bla.v1+json":
       reply.send(...);
       break;
  }
});