Preheat the oven to 350°F.

Melt 2 tablespoons of the butter in a large skillet over medium-high heat.

Add garlic and cook, stirring constantly, for 10 seconds.

Add leeks and cook until softened, about 5 minutes.

Add mushrooms and cook until softened and liquid has evaporated, about 10 minutes.

Remove from heat and stir in breadcrumbs, cheese, 1 teaspoon parsley, 1 teaspoon rosemary, 1 teaspoon sage, 1 teaspoon thyme, 1/4 teaspoon salt and 1/4 teaspoon pepper; set aside.

Combine remaining 1 tablespoon butter with remaining 1/2 teaspoon parsley, 1/2 teaspoon rosemary, 1/2 teaspoon sage and 1/2 teaspoon thyme; set aside.

Arrange turkey breast skin-side down on a clean surface so that it lies open and flat. Cover with plastic wrap, then pound lightly with a meat mallet to flatten and make an even thickness all over.

Discard plastic wrap and season turkey all over with remaining salt and pepper.

Spread stuffing over turkey, leaving a 3/4-inch border around the edge.

Close up snugly, tucking in the stuffing as you go, and then tie with kitchen twine at (1-inch) intervals around the entire turkey breast.

Rub turkey all over with reserved herb butter and arrange it in a roasting pan.

Roast uncovered, basting occasionally, until turkey reaches an internal temperature of 165°F, about 1 1/2 hours. (Cover with foil if top browns too quickly.)

Transfer turkey to a carving board, tent with foil and set aside for 15 minutes.

Remove and discard twine, slice turkey and serve.