For the potatoes, scrub the potatoes well, then poke holes in them with a fork or knife.
Put the water in the inner pot, set the steamer rack inside, and put the potatoes on the rack.
For the filling, in a stainless steel container that fits your Instant Pot, combine the onion, mushrooms, carrot, barbecue sauce, water, lentils, garlic, Barbecue Spice Blend, sugar if using, and salt. Stir to mix well.
Place the container on top of the potatoes. Cover with foil or a lid. Lock the lid into place, set the Pressure Release Knob to Sealing, select the Pressure
Cook/Manual setting at high pressure, and set the cook time to 17 minutes. Let the pressure release naturally, then open the lid. Remove the pan with the lentils and stir to mix. The mixture will continue to thicken as it sits. Taste for seasoning, adjusting if needed.
For the sriracha mayonnaise, stir together the mayonnaise and sriracha in a small dish, adding a bit of water, if needed, so it will be easy to drizzle.
When the potatoes are cool enough to handle but still warm, slice open. Brush the melted vegan butter on the cut potatoes and fill with the barbecue lentil mixture.
Drizzle with the sriracha mayonnaise, garnish with parsley and red pepper flakes, and serve warm.