Generate random on server site

I would like to generate a random value (for example number) on my server, but the clients (users) need to be sure about the randomness. It is possible? Which technologies or concepts do I need?

Example:

Client1 sends a request to my web server and send me some data.

Client2 sends a request to my web server and send me some data.

Client3... and so on.

Then I generate an "open" random and show it on my webpage. The crucial aspect is, that they all need one random but don't trust me.