Preheat oven to 350ºF. Grease a shallow 2-quart baking dish.
In a large skillet over medium heat, melt butter. Add celery and onions; cook until golden and tender. Dissolve bouillon cubes in hot water, then add to vegetables. Stir in salt, poultry seasoning and pepper; heat to boiling.
Add frozen potatoes to vegetables and heat stirring frequently, until potatoes are hot. Remove from heat; fold in toast cubes. Turn into baking dish. Cover and bake 30 minutes. Uncover and bake 15 minutes longer, until slightly crusty on top.