Heat olive oil in a skillet over medium high heat, add onions and cook until tender and translucent. Add garlic and cook for 1-2 minutes more. Reduce heat to low/medium and add the chopped spinach. Continue cooking until the spinach is very well wilted. Meanwhile, bring 4 cups of chicken broth and salt to a low boil in a large pot over medium high heat. Reduce heat to medium and add corn meal in a slow steady stream while whisking to prevent lumps. Reduce heat a bit more. Cook the corn meal, low simmer for about 40-60 minutes stirring often (every couple of minutes). Add the extra cup of chicken broth slowly as it begins to thicken. Taste the polenta to check for doneness. When you are satisfied with the consistancy, stir in pepper, butter, and parmesan cheese. Add in the spinach onion mixture and stir well, then transfer to serving dish. Let set for about 10 minutes, polenta will thicken as it cools.