Varnish would seal in the moisture, that is not what you want.
Couple of ideas
First you want to seal the ends mostly, any end grain will lose the moisture faster, and this will create stress in the wood, resulting in cracks.
Second idea, go by a couple of gallons of DeNatured Alcohol, DNA and then make up a tub, it can be made from plywood, lined with a large trash bag, for example, put the slab in there and then flood it with DNA, put a weight on it to stop it from floating so it is completely submerged, leave it for 24 to 48 hours then take it out and wrap the whole thing up in a large paper bag, or just wrap layers of brown paper over it, even newspaper will work. The DNA soaks into the wood and replaces the water in the wood, the DNA will then evaporate quicker, but more evenly so you should not get as much cracking, or at least that is the plan. Leave it wrapped up in the paper for 3 to 6 months.
Other ideas, get some paper, brown is good, and saturate the end grain with white wood glue, the water soluble stuff, then put the paper on the glue like you are wallpapering a wall, let it dry, this should slow down the end grain drying and stop stress cracks. Then down the road remove the glue and paper with a little warm water.
I hope this helps!
The pessimist complains about the wind; the optimist expects it to change; the realist adjusts the sails.
William Arthur Ward