Prepare the potatoes: Peel the potatoes (optional), then cut them into 1-inch cubes. Place them in the Instant Pot and cover with approximately 4 cups of water.
Cook: Lock the Instant Pot lid into place and set the Pressure Release knob to Sealing. Select Pressure Cook on High for 4 minutes. Press Start.
Quick release: When the Instant Pot beeps at the end of the timer, carefully switch the Pressure Release knob from Sealing to Venting (using a wooden spoon helps protect your hand from steam). When the float valve drops, open the lid.
Drain: Using a towel or potholders, carefully lift the inner pot and pour the potatoes into a colander. Let them drain for 1 to 2 minutes.
Mash: Return the drained potatoes to the inner pot. Mash with a potato masher until mostly smooth.
For traditional mashed potatoes: Add ¼ cup of plant-based milk, ¼ teaspoon of garlic powder, and ¼ teaspoon of kosher salt (optional). Stir until creamy.
For rosemary mashed potatoes: Follow the instructions for traditional mashed potatoes but add an additional ¼ cup plant-based milk, nutritional yeast, dried rosemary, an additional ¼ teaspoon garlic powder, and additional kosher salt to taste (optional). Serve warm.