Turn oven to 400.
Sift your 2 cups of cornmeal through a fine mesh strainer and discard the pieces that stay in the sifter-- you want a very fine grain for polenta. You should have roughly 1.5 cups once sifted.
Scrub your beets, cut off the top and tail end and peel with a vegetable peeler or sharp paring knife. Chop into uniform 1-1.5" chunks (if small, just halve or keep whole.) Toss the beet chunks in the oil and some salt and pepper. Spread on rimmed baking sheet (lined with parchment for easy clean up). Roast for 15 minutes, toss and reduce heat to 350, and roast until soft (another 20 minutes or so.) As soon as you remove them from the oven, toss with the maple syrup and vinegar. If you want, you can pop them back in the oven for 5-10 minutes to let the sweet stuff caramelize on the beets (but definitely use something to protect your pan so you don't have hard-candy beets stuck to it!)
While the beets are roasting, bring your 5 cups of stock to a boil. Bring an additional pot of water to a boil right beside it, 4-5 cups, so that in case the polenta gets too sticky you can add in a few ladlefuls of water to help thin it out as you work.
When the stock is boiling, add in the fine cornmeal. Don't worry if it clumps right away, just keep whisking and stirring constantly for about 10 minutes as you add all of it in. Continue to stir occasionally to prevent sticking, and reduce heat to medium low. Add in some water from your boiling pot as needed, and about 1 tsp of salt.
Right before serving, stir the goat cheese into the polenta and remove from heat. Taste to adjust seasoning--we ended up adding an extra splash of vinegar. Plate your dishes with some polenta on bottom, some of the caramelized beets on top, a few leaves of thyme and an additional small spoonful of maple syrup.