sudo iptables -t mangle -A FORWARD --match policy --pol ipsec --dir in -s 10.10.10.10/24 -o eth0 -p tcp -m tcp --tcp-flags SYN,RST SYN -m tcpmss --mss 1361:1536 -j TCPMSS --set-mss 1360

table mangle {
    chain FORWARD {
        # adjust MSS
        mod policy pol ipsec dir in saddr $PEER outerface eth0 proto tcp tcp-flags (SYN RST) SYN mod tcpmss mss 1361:1536 TCPMSS set-mss 1360;
    }
}

Error in /etc/ferm/ferm.conf line 46:
table mangle
{
    chain FORWARD
    {
        mod policy pol ipsec dir in saddr $ PEER outerface eth0 proto tcp tcp-flags <--
Unrecognized keyword: tcp-flags