Preheat oven to 400°. Combine flour and poultry seasoning. In another bowl, whisk egg and milk. Dip chicken, one piece at a time, in flour mixture, then in egg mixture; coat again with flour mixture.
In a large skillet, heat oil over medium-high heat. Brown chicken on both sides. Place in a greased 13×9-in. baking dish.
For sauce, combine cornstarch and 1-1/2 cups cold water in a large saucepan until smooth; stir in brown sugar, honey and soy sauce. Pulse rhubarb, onion, garlic and remaining water in a food processor until blended. Stir into cornstarch mixture. Bring to a boil over medium heat; cook and stir until thickened, 2-3 minutes. Pour over chicken.
Bake, uncovered, until a thermometer in the thigh reads 170°, 35-40 minutes.