In a medium saucepan, add the cubed potatoes and fill with cold water to cover the potatoes by 1-inch. Bring to a boil and cook until just fork-tender. Drain and set aside.
Set a large deep skillet over medium-high heat and add 1 tablespoon of butter. When the butter is melted, add carrots, celery, garlic powder, and poultry seasoning. Cook for 3 minutes, until vegetables are slightly tender. Stir in the peas and continue cooking for an additional 1 minute. Remove from heat and pour the mixture into a medium bowl.
In the same skillet used for the vegetables, heat the remaining 5 tablespoons of butter over medium heat. Add the onion and sauté for 2-3 minutes, until softened. Whisk in the flour, remaining 1/2 teaspoon black pepper, and celery seed. Gradually add the chicken-style broth and milk, 1/2 cup at a time. Reduce to low heat and simmer for 3-5 minutes, stirring occasionally until thickened.
Add the vegetable mixture to the sauce and stir to combine. Adjust seasoning to taste and serve hot over biscuits or toast.
Optional Crispy Topping: In a small bowl, combine the bread crumbs, melted butter, and garlic powder. Sprinkle over the top and serve.