Carefully wash the leeks and halve lengthwise. Cut the white of the leek into ¼-inch half rounds. Save the dark green parts to make a delicious vegetable stock.
Heat the grape seed oil in a Dutch oven.
While the oil is heating, season the bison cubes with salt and pepper.
Add the bison to the Dutch oven and brown for 2-3 minutes on one side, then flip to brown on the other side, adding the carrots, leeks, onion and celery.
Continue to brown the meat for an additional 3 minutes.
Pour in the stock and wine and add the whole thyme sprigs.
Bring to a simmer and simmer, covered for 2-2 1/2 hours.
Just before serving, remove the thyme sprigs and gently stir in the buerre manie.
Allow it to simmer for about a minute to thicken before removing from heat and serving.