Pour 1/2 cup water and 1/2 cup champagne into the Instant Pot and insert the steam rack.
Place the lobster tails on the steam rack, then secure the lid, making sure the vent is closed.
Using the display panel select the MANUAL or PRESSURE COOOK function*. Use the +/- keys and program the Instant Pot for 4 minutes.
When the time is up, quick-release the pressure, then remove the lobster tails to a shallow dish and cover with foil. Remove the rack and discard the water
Add 1 tbsp olive oil and 2 tbsp butter to the Instant Pot. Using the display panel select the SAUTE function.
When oil gets hot and butter has melted, add shallots and leeks to the pot and saute until soft, 2-3 minutes. Add garlic and cook for 1-2 minutes more. Add rice and cook for 2-3 minutes more.
Add champagne and stock to the pot and deglaze by using a wooden spoon to scrape any brown bits from the bottom of the pot.
Turn the pot off by selecting CANCEL, then secure the lid, making sure the vent is closed.
Using the display panel select the MANUAL or PRESSURE COOOK function*. Use the +/- keys and program the Instant Pot for 10 minutes.
Meanwhile, melt remaining 2 tsp butter in a small bowl. Crack open lobster shells, pry out the meat and roughly chop. Toss lobster meat with melted butter and 1 tsp lemon juice.
When cooking time is up, quick-release the pressure.
Add mascarpone, salt and while pepper. Stir until desired consistency is reached.
Serve risotto topped with lobster. Sprinkle with fresh thyme.