Add ground beef and onion to the Instant Pot. Using the display panel select the SAUTE function.
Cook and stir until no pink remains. Drain any liquids.
Add remaining Beef Mixture ingredients and stir to combine.
In a small bowl, melt >2 tbsp butter. Whisk in flour and beef broth until smooth. Add to pot and stir to combine.
Cook and stir for 3-4 minutes, until sauce begins to thicken.
Transfer the beef mixture to a 1 1/2 qt Instant Pot-friendly casserole and cover to keep warm.
Rinse out the pot, then combine 1 cup water and cubed potatoes in 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 COOK function*. Use the +/- keys and program the Instant Pot for 8 minutes.
When the time is up, quick-release the pressure.
Add remaining Potato Mixture ingredients to the pot and mash to desired consistency.
Top the beef mixture evenly with the mashed potatoes. Cover the casserole with foil.
Rinse out the pot, then add 1 1/2 cups of water and the steam rack.
Carefully lower the casserole onto the rack, then secure the lid, making sure the vent is closed.
Using the display panel select the MANUAL or PRESSURE COOK function*. Use the +/- keys and program the Instant Pot for 10 minutes.
When the time is up, quick-release the pressure.
Carefully remove the dish from the pot, top with cheese and let rest for 10 minutes before serving.
(Optional) Set under broiler for 5-7 minutes until cheese is lightly browned.
The Starch Factor
Substituting Yukon Gold potatoes for the russets changes the final texture into a dense paste rather than a fluffy mash. Russet potatoes contain less moisture and higher starch content, absorbing the butter and cheese without turning gluey. Cut the 1.5 pounds of tubers into uniform 1-inch cubes so they soften evenly during the 8-minute pressure cooking cycle. Drain the cooked potatoes completely before mashing to eliminate excess liquid that would otherwise seep into the beef filling below.
Pressure Management
Layering the components inside an electric multi-cooker requires strict adherence to timing. Brown the 93% lean ground beef thoroughly on the sauté setting, draining any accumulated grease to prevent a watery gravy. Whisk the 2 tablespoons of flour and beef broth into a smooth slurry before thickening. When steaming the assembled casserole dish on the rack for 10 minutes, use a quick-release method immediately when the timer sounds to prevent overcooking the vegetables.
Storage and Structure
Allow the finished dish to rest for 10 minutes uncovered so the starches set and the layers hold their shape when portioned. Stored in an airtight container within the refrigerator, this casserole maintains structural integrity for up to three days. Reheating individual portions in a conventional oven at 350 degrees Fahrenheit for 15 minutes restores the crisp edges of the cheddar cheese topping without turning the potatoes soggy.