Suitable and SECURE approach to give a call to existing On premise web API from SharePoint online

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New Contributor
We are consuming existing On premise web API from SharePoint online application. Currently we have used http call to consume that Web API, which is not enough to make a secure call. Please suggest suitable approach to secure On premise web API.
 
Note: We are not able to move our API on Azure because multiple application is consuming those APIs so please suggest suitable approach to manage the authentication part.