Melt butter or margarine in a skillet over medium heat. Cook onion, celery and garlic in butter, stirring frequently, until vegetables are soft (about 8-10 minutes). Stir in mushrooms.
Place stuffing mix in a very large bowl. Spoon cooked vegetables over stuffing. Add poultry seasoning, salt, pepper and tabasco. Pour in enough broth to moisten, and mix in eggs. Spray slow cooker crock with cooking oil. Transfer mixture to slow cooker, and cover.
Cook on High for 45 minutes, then reduce heat to Low, and cook for 4 to 8 hours. After four hours, check frequently to make sure stuffing stays moist. Add more broth as needed.
If using sausage, brown sausage, drain and stir into stuffing approximately one hour prior to serving. If you wish to use water chestnuts or oysters, add them when you add the canned mushrooms.
If you have decided to halve this recipe, it is VERY IMPORTANT that you reduce the time on high to 30 minutes and check it after three hours as a smaller slow cooker will cook hotter and faster. Whether you use one egg or two depends on whether you like your stuffing more or less with a "baked" consistency. When I halve it, I use two.