Heat the canola oil in a large saucepan over medium heat. Add the shallot and garlic and cook, stirring, until fragrant, 1 to 2 minutes. Add the chicken stock and milk and bring to a simmer, then slowly whisk in the polenta. Cook over medium heat until thick, stirring often, about 15 minutes. Reduce the heat and continue to cook and stir until the polenta is tender, about 5 minutes longer, adding stock or water if it gets quite thick. Stir in the Grana Padano and butter, and season to taste with salt and pepper. Cover and set aside to keep warm.