Add chicken, onions, ginger, lemon juice, and chicken stock to pressure cooker. Close and lock lid, set pressure cooker to "poultry", adjust time to 5 minutes, then press "start/stop". When done, natural release for 10 minutes, then do a quick release and make sure all pressure is released before opening lid. Carefully open lid and remove chicken. Transfer to platter and cover.
Add oranges, honey, soy sauce, and tarragon to pressure cooker.
In a bowl, combine water and cornstarch. Whisk to make a slurry. Pour mixture into cooker. Mix well. Set pressure cooker to "sear", adjust time to 15 minutes, and press "start/stop". Bring sauce to a simmer to thicken.
Once sauce has reached desired consistency, serve chicken over cooked udon noodles. Pour sauce over chicken. Garnish with scallions.
