Bring 1/2 teaspoons kosher salt and 1 quart water to a boil in a 3-qt. saucepan over high heat. Stir in rice, and return to a boil. Reduce heat to medium; cover and cook 30 minutes.
Meanwhile, melt butter in a heavy saucepan over low heat; whisk in flour until smooth. Cook, whisking constantly, 1 minute. Gradually whisk in milk and next 2 ingredients. Increase heat to medium, and cook, whisking constantly, 3 to 4 minutes or until mixture is thickened and bubbly. Stir in 1/4 teaspoons each kosher salt and black pepper.
Cook ham in 1 tablespoons hot oil in a 12-inch cast-iron skillet over medium-high heat, stirring occasionally, 6 minutes or until beginning to brown. Stir in onion and next 2 ingredients. Cook, stirring occasionally, about 6 minutes or until onions are tender. Stir in garlic, and cook 1 minute. Add sherry, and cook, stirring constantly, 1 minute or until sherry is evaporated. Remove mixture from skillet.
Preheat oven to 375°. Sprinkle chicken with remaining 1/2 teaspoons each salt and pepper. Add remaining 1 tablespoons oil to skillet. Cook chicken in hot oil over medium-high heat 4 minutes on each side or until brown. Remove skillet from heat; transfer chicken to a plate.
Drain rice. Stir together rice, ham mixture, and sauce in skillet. Place chicken on top of rice mixture.
Bake at 375° for 30 minutes or until mixture is bubbly and chicken is done. Let stand 10 minutes before serving.