Preheat your oven to 375°F (190°C). Heat the olive oil in a large 12-inch oven-safe skillet over medium-high heat. Add the chicken pieces and cook for 4-5 minutes until golden brown and just cooked through. Remove the chicken and set aside.

Reduce heat to medium. In the same skillet, add the diced onion, carrots, and celery. Sauté for 5-7 minutes until the vegetables start to soften. Add the minced garlic and cook for another minute until fragrant.

Add the butter to the skillet and let it melt. Sprinkle the flour over the melted butter and stir constantly for 1-2 minutes to create a roux.

Gradually whisk in the chicken broth and milk, stirring constantly to prevent lumps. Bring to a simmer and cook for about 5 minutes until the sauce thickens enough to coat the back of a spoon.

Stir in the thyme, rosemary, salt, and pepper. Add the cooked chicken and frozen peas, stirring until well combined. Adjust seasoning if needed.

Transfer the skillet to the oven and bake for 20-25 minutes until bubbly and lightly golden. Let it rest for 5 minutes before serving.