Add chicken to the cooking pot on top of onions in a single-ish layer.
Sprinkle Italian seasonings and salt over chicken.
Spoon pesto sauce evenly over chicken.
Lock lid in place and close pressure valve. Cook at high pressure for 3 minutes. When beep sounds, wait 5 minutes and then release the rest of the pressure.
Slice 3/4 of Mozzarella cheese into thin slices and grate or cut the rest into slivers.
Remove lid and stir onions and chicken into sauce to combine.
Place Mozzarella slices over chicken.
Slice tomatoes and place tomato slices on top of cheese.
Turn off pressure cooker and place Mealthy Crisplid over pot and set temperature to 325 and time to 2 minutes. Keep an eye on the cheese and remove lid once cheese is browned and bubbly, or replace pressure cooker lid loosely over pot and let sit for 2-4 minutes, or until cheese has melted.
Plate chicken and garnish with Mozzarella slivers.
If using chicken thighs, add one minute to cook time.Make sure to cut all the fat off the chicken.Grape tomatoes can be used in place of Roma tomatoes. They can be added in with the chicken or after cooking with the Mozzarella.