SEASON CHICKEN: Sprinkle salt and pepper inside chicken cavity and add a couple chunks onion and lemon if desired. Combine all seasoning ingredients in a small bowl. Rub 2/3 of it on chicken, including under the skin of chicken breasts (gently loosen skin from flesh, then smear it).
PREPARE FOR INSTANT POT COOKING: Put broth, onions and garlic clove into the bottom of the pot (6-8 quart instant pot). Place the trivet that comes with pot over the mixture. Place chicken on top, chicken breast-side up.
INSTANT POT SETTINGS/COOKING: Place on instant pot lid and turn clockwise to lock. Turn valve to SEALING position. Push HIGH PRESSURE or MANUAL button. Use + or - buttons to set time. Calculate time at 6 minutes per pound (450 g) LESS 1 minute. So a 3.5 lb/1.59 kg chicken will take (3.5 x 6) - 1 = 20 minutes. Therefore set time to 20 minutes. Pressure will take 10-15 minutes to build before the pressure clock starts. When finished (instant pot will beep), wait 15 minutes for a NATURAL PRESSURE RELEASE, then do a quick release if necessary for the pressure button to pop down.
HEAT OVEN to 500F/260C while chicken is cooking in instant pot. Prepare a pan, lined with aluminum foil and sprayed with oil. If desired, add veggies for roasting on half the pan, tossing them with olive oil, salt and pepper.
FINISH CHICKEN IN OVEN (WITH VEGGIES if using): Remove chicken from instant pot (I use two forks at either end - don't pierce meat). Place on prepared pan along side veggies if using. Brush remaining seasoning on chicken skin. Roast for 10 minutes until skin browns. Instant thermometer should show an internal temperature of 165F. Cut up chicken on cutting board and serve with gravy.
MAKE GRAVY: (Note 2 to make directly in instant pot). While chicken is roasting in oven, discard onions and garlic from instant pot. Pour broth into a separator (or skim grease from broth and reserve). Put 2 tablespoons grease in separate saucepan. Heat to medium heat. Add flour and stir for 2 minutes. Add 2 cups broth (juices) from the instant pot. Continue cooking for a few minutes until thickened. Taste and adjust seasonings, adding thyme, salt, pepper as desired. Add more broth if needed to desired consistency.