Set the Instant Pot to the Sauté function. Add the olive oil and cubed chicken breast, searing until the edges are golden brown.
Add the diced yellow onion, minced garlic, and cubed pumpkin to the pot. Sauté for another 3 minutes to develop a roasted flavor profile.
Stir in the tomato paste, dried oregano, sea salt, and black pepper, ensuring the chicken and pumpkin are evenly coated.
Pour in the chicken bone broth and stir to deglaze the bottom of the pot, scraping up any browned bits.
Secure the lid and set the Instant Pot to Manual/Pressure Cook on High for 4 minutes.
Once the timer ends, perform a Quick Release of the pressure to prevent the pumpkin from overcooking.
Remove the lid and add the zucchini slices, cauliflower florets, and broccoli florets. These delicate vegetables should not be pressure cooked.
Select the Sauté function again and simmer the stew uncovered for 5 to 7 minutes. This will steam the green vegetables to a tender-crisp texture while reducing the broth into a thick, rich sauce.
Serve immediately over high-fiber toast or your favorite pasta, or portion into containers for freezing.