Pat chicken dry, and sprinkle with salt and pepper. Select SAUTÉ setting on a programmable pressure multicooker (such as Instant Pot). (Times, instructions, and settings may vary according to cooker brand or model.) Select HIGH temperature setting, and allow to preheat 2 to 4 minutes. Add oil to cooker. Add half of chicken, meat side down, and cook, undisturbed, until golden brown, about 5 minutes. Transfer to a plate, and repeat with remaining chicken. Set chicken aside.

Add garlic, rosemary, and thyme sprigs to cooker; cook, stirring constantly, until garlic is fragrant and softened, about 1 minute. Add lemon zest and juice, and stir to loosen any browned bits from bottom of cooker. Stir in broth and mustard. Return chicken to cooker, cover cooker with lid, and lock in place. Press CANCEL. Turn steam release handle to SEALING position. Select MANUAL/PRESSURE COOK setting. Select HIGH pressure for 9 minutes. (It will take about 10 minutes for cooker to come to pressure before cooking begins.)

When cooking has finished, carefully turn steam release handle to VENTING position, and let steam fully escape (float valve will drop). (This will take 2 to 3 minutes.) Remove lid from cooker. Transfer chicken to a platter.

Select SAUTÉ setting. Select HIGH temperature setting, and allow sauce to reduce by half, stirring occasionally, about 5 minutes. Press CANCEL. Gradually stir butter into sauce in cooker until melted and combined. Pour over chicken. Garnish with lemon slices and thyme sprigs.