Combine the onion, garlic, carrots, celery, mushrooms, red lentils, water, chickpeas, thyme, rosemary, soy sauce, 1 teaspoon salt, and several grinds of pepper in the Instant Pot and give it a stir. 


Arrange a 2.5-inch trivet over the vegetables and place a 7-inch oven-safe bowl on top. Add the cauliflower to the bowl. Secure the lid and move the steam release valve to Sealing. Select Manual/Pressure Cook to cook on high pressure for 5 minutes. 


When the cooking cycle is complete, let the pressure naturally release for 10 minutes, then move the steam release valve to Venting to release any remaining pressure. When the floating valve drops, remove the lid and use oven mitts to lift the trivet and the bowl out of the pot. 


Stir the filling at the bottom of the pot. Taste and adjust the seasonings as needed. The lentils should dissolve into the sauce as you stir, thickening it. Pour the cooked cauliflower into a large bowl and use a fork or potato masher to mash it until smooth. Generously season the mash with salt and pepper. 


To serve, spoon the warm filling into bowls and top each with mashed cauliflower. Garnish with fresh herb sprigs and a final grind of pepper. Store leftovers in two separate airtight containers—for the filling and the cauliflower mash—in the fridge for 5 days.