I either...
A: Go to twister side control and get way up under the arm then step over while controlling the arm of the side I had side control on. This either gets them in a good position for the armbar, or lands me in a solid technical mount.
B: Or I go to reverse kesa and control the far side arm. I then step over and immediately switch to s-mount toward the side I started. This usually prevents them from grabbing the leg as I come over, and if they try I can usually crank on a kimura.
The obvious answer to this is not to try to mount when your opponent is on their side. That's how most of our higher belts get out of side control lol; they just chill out and pretty much beg you to mount, trap the leg and follow over to your back. It's a super bummer to go from the top to having your back taken