Cook the polenta according to the package directions. Transfer the polenta to a wooden cutting board or baking sheet lined with parchment paper and spread it 1/2 inch thickness. Let cool until set, at least 45 minutes.
Preheat the oven to 350°F (177°C). Lightly butter a 9-by-13-inch (23-by-33-cm) baking dish.
In a large skillet over medium heat, warm the oil. Add the sausages and cook until browned, 5 to 10 minutes.
Add the onion and cook until softened, about 5 minutes. Stir in the stock or broth, half the chopped herbs, and salt and pepper to taste. Remove from the heat.
Cut the polenta into slices about 1/2 inch (12 mm) thick. Arrange a layer of polenta in the prepared baking dish. Cover the polenta with half the sausage mixture, half the Taleggio, and half the Parmesan. Repeat with the remaining polenta, sausage mixture, Taleggio, and Parmesan. Sprinkle with the remaining herbs. If desired, top with 1 to 2 tablespoons butter cut into small pieces.
Bake until brown and bubbling, about 40 minutes. Serve warm.