Bootstrap.cs

public class Bootstrap : ApplicationEventHandler
{
  protected override void ApplicationStarted(UmbracoApplicationBase umbracoApplication, ApplicationContext applicationContext)
  {
    ExamineManager.Instance.IndexProviderCollection["ExternalIndexer"].GatheringNodeData += (sender, args) =>
    {
      ...
    
      // Extract JSON properties
      var fieldKeys = args.Fields.Keys.ToArray();
      foreach (var key in fieldKeys)
      {
          var value = args.Fields[key];
          if (value.DetectIsJson())
          {
              IndexNestedObject(args.Fields, JsonConvert.DeserializeObject(value), key);
          }
      }
      
      ...
    }
  }
  
  private void IndexNestedObject(Dictionary<string, string> fields, object obj, string prefix)
  {
      var objType = obj.GetType();
      if (objType == typeof(JObject))
      {
          var jObj = obj as JObject;
          if (jObj != null)
          {
              foreach (var kvp in jObj)
              {
                  var propKey = prefix + "_" + kvp.Key;
                  var valueType = kvp.Value.GetType();
                  if (typeof(JContainer).IsAssignableFrom(valueType))
                  {
                      IndexNestedObject(fields, kvp.Value, propKey);
                  }
                  else
                  {
                      fields.Add(propKey, kvp.Value.ToString().StripHtml());
                  }
              }
          }
      }
      else if (objType == typeof(JArray))
      {
          var jArr = obj as JArray;
          if (jArr != null)
          {
              for (var i = 0; i < jArr.Count; i++)
              {
                  var itm = jArr[i];
                  var propKey = prefix + "_" + i;
                  var valueType = itm.GetType();
                  if (typeof(JContainer).IsAssignableFrom(valueType))
                  {
                      IndexNestedObject(fields, itm, propKey);
                  }
                  else
                  {
                      fields.Add(propKey, itm.ToString().StripHtml());
                  }
              }
          }
      }
  }
  
}

StringExtensions.cs

public static class StringExtensions
{
  public static bool DetectIsJson(this string input)
  {
      input = input.Trim();
      if (input.StartsWith("{") && input.EndsWith("}"))
          return true;
      if (input.StartsWith("["))
          return input.EndsWith("]");
      return false;
  }
}