Prepare the Chicken: Pat chicken dry with paper towels and sprinkle with salt and pepper. Using the highest sauté or browning function, heat 1 tablespoon of oil in the Instant Pot until just smoking. Place half the chicken, skin side down, and cook until well-browned, 6 to 8 minutes; transfer to a plate. Repeat with the remaining oil and chicken.
Cook the Aromatics: Add fennel to the fat left in the pressure cooker and cook using the highest sauté or browning function until softened for 3 to 5 minutes. Stir in garlic, tomato paste, saffron, and cayenne, and cook until fragrant, about 30 seconds. Stir in the flour and cook for 1 minute. Slowly whisk in wine and pastis, scraping up browned bits.
Assemble the Stew: Stir in chicken broth, tomatoes, potatoes, and orange zest. Nestle the chicken, skin side up, into the pot, ensuring it is surrounded by the liquid. Add any accumulated juices from the chicken.
Pressure Cook: Lock the lid in place and close the pressure release valve. Select the high-pressure cook function and cook for 3 minutes. Turn off the Instant Pot and use the quick-release function to release the pressure. Carefully remove the lid, allowing steam to escape away from you.
Finish & Serve: Transfer chicken to a plate and discard the skin, if desired. Remove and discard the orange zest. Let the cooking liquid settle, then skim excess fat from the surface using a wide, shallow spoon. Adjust consistency with extra hot broth, if needed. Stir in tarragon or parsley and season with additional salt and pepper to taste. Divide the stew between shallow bowls, top with chicken, and serve.