In a large dutch oven, melt 2 tablespoons of the butter over medium-high heat. Add the onion, carrots and celery. Cook until the veggies are just tender, about 8-10 minutes. Add the garlic and cook for an additional minute. Add the broth, potatoes, bay leaf, a heavy dose of salt and pepper and Italian seasoning. Mix to combine. Bring the mixture to a boil and then lower the heat to a simmer. Cook until the potatoes are fork tender, about 15 minutes. Add the turkey breast, frozen peas, and fresh parsley. Stir to combine and lower the heat to low.
In a separate saucepan, melt the remaining 6 tablespoons of butter over medium heat. Whisk in the flour and cook until the raw flour has cooked out and the mixture is golden. Continuing to whisk, pour in the whole milk, half and half and sour cream. Bring the mixture to a simmer, whisking constantly, until the mixture has thickened substantially.
Pour the flour mixture into the soup and stir to combine. Keep warm on the stove, covered, while you prepare your buttermilk biscuit croutons.
Prepare your buttermilk biscuit croutons by preheating an oven to 450 degrees F and lining a large baking sheet with parchment paper or a silicone baking mat. Set aside.
In a large bowl, whisk together the flour, baking powder, salt and sugar. Set aside.
In the pitcher of your blender or food processor, add the butter. Add the flour mixture on top. Cover with the lid and pulse 8-10 times until coarse crumbles have formed (If you don''t have a blender or food processor, just use your hands and crumble until coarse crumbles form - This will take a couple of minutes). Transfer back to the bowl and add the cheddar cheese and black pepper. Mix to combine.
Create a well in the middle of your flour mixture, and add the buttermilk. Fold the ingredients together until a soft dough forms. Do not over-mix.
In teaspoon-sized dollops, drop the biscuits onto the prepared baking sheet. Fill the baking sheet, making sure to not overcrowd the pan. You may need to bake these biscuits in two batches. Bake at 450 degrees F for 8-10 minutes or until the tops are just beginning to brown. Remove from heat and serve immediately with turkey pot pie soup.