Heat the oven to 375°F and grease a 9 x 13-inch baking dish with a little unsalted butter.

Spread the dry bread cubes on a rimmed baking sheet and bake for 8 to 10 minutes, until dry and lightly golden.

Melt 4 tablespoons unsalted butter in a large saucepan over medium heat, then cook the yellow onion and celery for 6 to 8 minutes, until softened.

Stir the all-purpose flour into the vegetables for 1 minute.

Gradually whisk in 1 1/2 cups low-sodium chicken broth and the whole milk, then simmer for 3 to 4 minutes, stirring, until the sauce coats a spoon.

Stir in 1/2 teaspoon dried thyme, the fine salt, and ground black pepper.

Fold the cooked chicken and frozen peas into the sauce and spread in the prepared baking dish.

Melt the remaining 2 tablespoons unsalted butter and toss with the toasted dry bread cubes, remaining 1/2 cup low-sodium chicken broth, remaining 1/2 teaspoon dried thyme, and dried rubbed sage.

Spread the moistened bread mixture evenly over the filling without pressing it down.

Bake uncovered at 375°F for 25 to 30 minutes, until bubbling at the edges, golden on top, and at least 165°F in the center.

Let stand for 10 minutes before dividing into 6 portions.