Preheat oven to 375℉. Cover baking sheet is tin foil if desired. Place cauliflower rice on counter to thaw.
Prepackaged butternut squash cubes typically come in larger chunks. I like to cut them down to ½ inch cubes for faster cooking. Place cubes on baking sheet and drizzle with oil. Use a spatula to mix until all cubes are coated. Sprinkle evenly with 2-3 pinches of salt and a couple dashes of black pepper. Bake 15-20 minutes, flipping at 10 minutes, until fork tender and slightly caramelized. Let cool about 5 minutes.
While squash is cooking, dice onions, celery, and garlic. Gather your ingredients. Make the spice blend if desired. Cut broccoli into florets if necessary.
When squash is cooled to the touch, place it in a blender or food processor. Add in nutritional yeast, Dijon mustard, chicken broth, and salt. Blend until completely smooth about 1-1½ minutes.
Select SAUTE function on the Instant Pot and add oil. When oil is melted and pot is warm add diced onion and celery with 2 pinches of salt. Cook stirring often until softened about 5 minutes.
Add minced garlic, salt, black pepper, ground mustard, thyme, rosemary, and bay leaf. Cook stirring constantly until spices are fragrant.
Add vegetable broth to stop the spices from cooking. Add butternut squash "cheese" sauce and mix to combine. Add whole chicken breasts, broccoli florets and cauliflower rice.
Close the lid. Make sure the vent is sealed. Select PRESSURE COOK (high pressure) 10 minutes. After cooking, allow pressure to naturally release for at least 10 minutes or longer. Release any remaining steam, making sure your skin is protected with a oven mitt or dish cloth.
Remove chicken to a cutting board and dice or shred to bite sized pieces. Add coconut milk to Instant Pot and stir to combine. Add back in diced chicken. Serve warm.