Pour chicken broth into a multi-functional electric pressure cooker. Place a steamer rack into the pot and add yam and potato. Sprinkle pink salt on top. Lock the lid and seal. Set pressure to high for 8 minutes and allow pressure to build (10-15 minutes).

Release pressure carefully using quick release for 5 minutes, unlock and remove lid. Remove yam, potato, and rack; strain liquid into a glass and reserve. Return yam and potato to the pot and add milk and butter; mash until smooth. Transfer mashed topping to a bowl.

Preheat oven to 350°F (175°C).

Select Saute function on pressure cooker, heat oil on high. Add onion and garlic; saute for 1-2 minutes until tender. Add ground beef, salt, and pepper, stir well for 2-3 minutes. Add mushrooms and cook for 1 minute.

Stir corn, peas, broccoli, carrot, and celery into the pot with reserved liquid. Lock lid and set to Manual, low pressure for 3 minutes, allowing pressure to build (5-10 minutes).

Release pressure using quick release for 5 minutes and unlock lid. Strain excess liquid and suit filling to Saute mode.

Dissolve gravy powder in water and add to the filling in the pot. Cook and stir until gravy thickens, about 2 minutes. Transfer filling to baking dish, cover with mashed topping and cheddar.

Bake in the preheated oven until cheese melts, approximately 10 minutes.