Add vegetable stock to the Instant Pot.
The aromatics go in next. Add the lemongrass, green onion whites, lemon leaves and ginger.
Add chopped baby corn, carrots and mushrooms.
Add salt, sugar and black pepper powder. Mix well.
Close the Instant Pot lid and set the mode to pressure cook. Turn the valve to sealing and time to 5 minutes.
Once the cooking is complete, wait for 2 minutes and then release all the pressure in small successions till the valve drops.
Open the Instant Pot and fish out the lemongrass and lime leaves.
Add the tomatoes, broccoli, chopped cucumber and firm tofu.
Add in the tom yum paste and soy sauce. Mix well.
Turn the Instant Pot mode to sauté and let the soup simmer for 4-5 minutes.
Turn the Instant Pot off and serve the soup immediately. Top it with chopped cilantro and green onions.