Need some help with routing multiple subnets over IPsec tunnel

I am not a networking expert by any means. I know just enough to know when I've truly broken something. That being said...

I currently have a fully functional IPsec tunnel connecting a fortigate (main office) and a mikrotik (branch office). The branch office has only one subnet, 192.168.100.0/24 but the the main office has several. The existing tunnel connects to the main office subnet 10.0.0.0/22 but I need it to also reach some other subnets. All of these other subnets are fully routed on the main office side, but I am only getting the one subnet over the IPsec. From what I have read, this should be expected.

If I want to have a one to many type of tunnel, can I get there with just IPsec? Do I need to make a separate tunnel for each subnet I want to connect to? Or do I need to use something like OSPF or GRE?