Heat the olive oil in a large skillet over medium heat. Add the sausages and cook, turning once, until golden, about 6 minutes. Add the garlic and cook, stirring, 1 minute. Add the wine, scraping up any browned bits from the bottom of the skillet. Add the grapes and spread the mixture into an even layer. Cover and continue to cook until the sausages are cooked through, about 10 minutes. Remove the sausages to a plate; cover with foil to keep warm.
Increase the heat to high and continue cooking the grapes until they burst and the sauce is reduced, about 12 minutes. (Mash the grapes slightly with a fork if they¿re not bursting.) Remove from the heat and stir in the vinegar; season with salt and pepper.
Meanwhile, bring the milk and 2 cups water to a boil in a medium saucepan. Whisk in the polenta and thyme. Reduce the heat to low and cook, whisking often, until the polenta thickens, 3 to 5 minutes. Remove from the heat and stir in the fontina until smooth; season with salt and pepper.
Stir the parsley into the grape sauce and slice the sausages. Top the polenta with the sausages, grape sauce and more parsley.