Saute the onion in 2 Tbsp of oil until lightly golden (you don't have to do this step, however it will add more flavor to the pesto).

In a small food processor, add 1/4 cup oil, sauteed onions, tomatoes, seasoning and garlic. Blend until smooth. Squeeze in a 2 Tbsp of the lemons juice, add walnuts and salt, pulsing a few times to chop them fine (don't over process into a paste).

Put chicken and marinade into a Ziploc bag, mixing until chicken is thoroughly coated. Chill for 2-24 hours.

For skinless chicken - place directly into pressure cooker. For chicken with skin - scrape marinade off chicken (reserving), and sear both sides in a hot skillet with 2 Tbsp of oil, about 3 minutes/side.

Put rack into the pressure cooker, then pour in water/broth. Add chicken in a single layer, and spoon all of the marinade over top to cover chicken. Arrange lemon wedges (even those you squeezed) around chicken and set timer - each model is different, approx 25-35 minutes.

Once cooked, place chicken on a baking pan and broil for 5-8 minutes - this allows the nuts in the topping to brown, adding more flavor.

If desired, pour liquid from pressure cooker into a skillet and bring to a boil. Reduce by half, then drizzle over chicken to serve.