Preheat oven to 400 degrees.Peel and core the onions. Take a thin slice off the bottom if necessary to make them sit upright and level. Starting in the center, make slices in the onion to the outer edges but not slicing all the way through the onion. Place the onions in a baking dish. Fill each onion with 1/4 Teaspoon pepper, 1 Teaspoon beef bouillon, 1/2 Teaspoon soy sauce, 1/2 Teaspoon Worcestershire sauce and 1 Tablespoon of butter. Cover with foil, bake at 400 degrees for about 30 minutes. All ovens are different, check the onions at the 15 minute mark to see how they're doing. When the onions are cooked through, place 1 Tablespoon of grated Parmesan cheese on top of each onion, turn the oven and off , place the pan back into the oven until cheese has melted.
The salt in the bouillon and soy sauce is enough for this recipe. To cut down even more on salt try using low sodium soy sauce and bouillon.