jest testing function call inside another function

Is it possible to test startServer() call once launcApp is called?

import { Server } from './Server/Server';

export class Launcher {
    
    private server: Server;

    public constructor() {
        this.server = new Server();
    }
    public launchApp() {
        this.server.startServer();
    }

}

I tried to do

Server.prototype.startServer = jest.fn();
new Launcher.launchApp();
expect(Server.prototype.startServer).toBeCalled();

but it fails