Melt butter in a soup pot over medium heat. Add onions, mushrooms, and garlic. Cook until onions are translucent. Add mushroom soup, milk, seasonings (generally go heavier on pepper), sausage, and potatoes. Cook on medium-low until well blended and small bubbles appear. Reduce heat to low and cook for 10 minutes. Add crab (if using), shrimp, and corn. Cook for a few minutes, or until heated through. Remove promptly from heat and serve.