Monitoring the proper functioning and performance of an infrastructure spanning multiple Cloud datacentres is challenging. It requires continuously aggregating monitored data across multiple source machines and processing them so to obtain useful alerts and insights. In this article, we propose a simple open-source prototype tool to perform highly customisable fault and performance monitoring across multiple Clouds. Differently from commercial tools, our prototype is simpler to deploy and it can be configured through a declarative approach, by simply specifying data monitoring tasks and aggregation policies. We illustrate such peculiarities over a use case relying on three datacentres under the Italian Research and Education Network Consortium.