Roast the Chicken: Begin by preheating the oven to 400°F and lining a medium baking sheet with foil. Arrange the split chicken breasts on the prepared baking sheet, drizzle with olive oil, and season with salt, black pepper, 1 teaspoon of Italian seasoning, 1 teaspoon of granulated onion, and the granulated garlic. Roast the chicken for 45-50 minutes, until the skin is golden and the internal temperature reaches 165°F.

Prepare the Vegetable Filling: While the chicken roasts, melt butter in a large skillet or medium pot over medium-high heat. Add the sliced leeks, diced carrots, celery, and parsnip, along with a pinch of salt and pepper. Stir in the remaining 1 teaspoon of Italian seasoning and ground white pepper. Sauté the vegetables for about 10 minutes until they begin to soften.

Thicken the Sauce: Add the pressed garlic and cook until aromatic. Sprinkle in the flour and stir for about 30 seconds to eliminate the raw taste, creating a thick paste. Gradually add the chicken stock while whisking to ensure smooth incorporation. Then, pour in the half-and-half and continue to whisk until fully combined. Bring the mixture to a simmer, then reduce the heat and let it cook for 22 minutes, stirring occasionally until the vegetables are tender.

Shred the Chicken: Once the chicken is roasted, remove it from the oven and let it cool slightly. Increase the oven temperature to 425°F. Once cool enough to handle, discard the skin and bones, then cube the meat into bite-sized pieces.

Finish the Filling: Add the cubed chicken, green peas, chopped parsley, thyme leaves, and the remaining ½ teaspoon of granulated onion to the sauce. Stir to combine, ensuring the chicken is evenly distributed. Transfer the mixture into a greased medium-large baking dish (approximately 9 x 11 ½ inches). Set aside while preparing the biscuit topping.

Prepare the Biscuits: In a mixing bowl, combine the flour, baking powder, and salt. Whisk together, then add the half-and-half and melted butter. Stir gently with a spatula until the mixture forms a thick batter. Be careful not to overmix to ensure the biscuits remain light and fluffy.

Assemble and Bake: Using a 2-tablespoon scoop, dollop about 8 mounds of biscuit batter evenly over the chicken mixture, gently pressing them down to flatten any peaks. Lightly brush the biscuits with a touch of half-and-half. Place the baking dish on a foil-lined baking sheet and bake for 24 minutes, or until the biscuits are golden brown and fully cooked. Optionally, cover loosely with foil during the last 8 minutes of baking to prevent the biscuits from overbaking.

Rest and Serve: Allow the chicken pot pie to rest for 15 minutes before serving. This helps the biscuits set and cools the filling to a comfortable temperature. Garnish with fresh thyme leaves, then serve and enjoy.