Preheat oven to 350 degrees. In a large ovenproof skillet, heat 1 teaspoon oil over medium heat. Add leeks, and cook 4 minutes. Add 2 teaspoons each rosemary and thyme; cook 1 minute more. Season with 1/4 teaspoon salt and 1/2 teaspoon pepper. Transfer to a medium bowl; let cool.
Using a sharp knife, carve a pocket in the center of each chicken breast by inserting the tip into the thickest part and cutting an opening (leaving about 1 inch uncut on each side), being careful not to cut all the way through the breast. Place 2 tablespoons leek mixture in each pocket. Heat 1 teaspoon oil in a large skillet over medium-high heat. Add chicken breasts, and saute until golden, about 3 minutes on each side. Remove from heat, and transfer to clean work surface.
Place bread in the bowl of a food processor fitted with the metal blade, and process until fine crumbs form. Transfer breadcrumbs to a bowl. Add remaining teaspoon rosemary and 1 teaspoon thyme. Season with the remaining 3/4 teaspoon salt and 1/2 teaspoon pepper. Spread 1 teaspoon mustard over one side of each chicken breast, and cover with about 3 tablespoons herbed breadcrumbs, pressing gently to adhere. Drizzle remaining teaspoon oil over all.
Place skillet in oven; roast chicken until it is golden and cooked through, about 15 minutes. Remove from oven; transfer chicken to serving dish, and cover with foil while you make the sauce. Place skillet over medium heat, and add chicken stock; deglaze pan, stirring to loosen any browned bits with a wooden spoon. Continue cooking until stock is reduced by half, about 3 minutes. Remove from heat; add remaining teaspoon thyme. Serve with the chicken.