In a large deep pan, bring the vegetable broth to a boil. Add the polenta and stir with a whisk or a long spoon. Reduce heat to low and simmer gently, stirring constantly until the polenta is thick and has soaked in the broth, about 15 minutes. Use a long spoon as the mixture will pop and bubble while cooking, and can burn. When polenta is thick, add the nutritional yeast and salt. Stir and set aside. Add lemon juice; stir.