2 cups self-rising all-purpose flour
3/4 stick (6 tablespoons) cold unsalted butter, cut into 1/2-inch pieces
1 cup whole milk
All-purpose flour for dusting
For chicken in gravy
1 medium onion, finely chopped
1 carrot, finely chopped
1 celery rib, finely chopped
1/4 teaspoon salt
3/4 teaspoon black pepper
3 tablespoons unsalted butter
2 tablespoons all-purpose flour
1 3/4 cups reduced-sodium chicken broth (14 fl oz)
1/2 teaspoon chopped fresh thyme
1/2 cup half-and-half
3 cups chopped cooked chicken (from a 3- to 3 1/2-lb rotisserie chicken)
1/2 teaspoon fresh lemon juice
Put oven rack in middle position and preheat oven to 450°F.
Blend together flour and butter in a bowl with a pastry blender or your fingertips until mixture resembles coarse meal with some small (roughly pea-size) butter lumps. Stir in milk with a fork just until a sticky dough forms.
Turn dough out onto a floured surface and gently knead 7 or 8 times. (Do not overwork, or biscuits will be tough.) Pat dough into a 9- by 6-inch rectangle on a lightly floured surface, then cut into 6 (3-inch) squares. Arrange squares 1 inch apart on an ungreased baking sheet and bake until golden brown, 15 to 18 minutes. Cool to warm on baking sheet on a rack.
Make chicken in gravy while biscuits bake:
Cook onion, carrot, celery, salt, and pepper in butter in a 10-inch heavy skillet over moderate heat, stirring occasionally, until vegetables are softened, 8 to 10 minutes.
Sprinkle flour over vegetables and cook, stirring, 1 minute. Stir in broth and thyme and bring to a boil, stirring, then boil over moderate heat, stirring occasionally, 2 minutes.
Add half-and-half and chicken to gravy and gently simmer until chicken is heated through, about 3 minutes. Stir in lemon juice.
Halve biscuits horizontally and put each bottom half in a shallow bowl. Spoon chicken in gravy over biscuit bottoms and cover with biscuit tops.