Turn the Instant Pot on to saute mode and add the oil. Saute the onions and celery for about 5 minutes.Add the 2 tsp chicken base, stir to melt the chicken base into the veggies. Pour in 2 cups water.
Turn saute mode off
Add the next 4 ingredients in this order: rice, raw chicken breast cubes, mushrooms.Sprinkle with 1 tsp kosher salt and 1/2 tsp black pepper.
Put the lid on the pot and lock it into place. Turn the top pressure vent to "seal". Set to cook on Rice mode for 12 minutes.
When the Instant Pot beeps do a quick release by turning the top pressure valve to "vent".
Add the 2 heaping cups of frozen broccoli florets and stir into the rice (I find using a fork for this works best because it "fluffs" the rice)Sprinkle the drained chopped water chestnuts over top. Close the lid of the pot and let the residual heat in the pot thaw the broccoli.
Whisk together the cream/cheese sauce ingredients: 1 cup shredded cheese, 1/2 cups milk, 1/3 cup mayonnaise, 1/3 cup non-fat plain greek yogurt, 1/2 tsp garlic powder.
Pour the cheese sauce over the chicken and rice and stir to coat.