Season the Sirloin steak cubes on all sides with salt, pepper, garlic powder, onion powder, paprika, thyme, and rosemary.
Place the Inner Liner pot inside the Instant Pot. Make sure the Instant Pot is plugged in. Press the "Saute" button on the front of the Instant Pot. Wait until the display says "Hot" and then add 1 Tablespoon oil to the Inner Liner.
Wait until the oil is hot and shimmering, then add the cubed sirloin beef tips, and cook for 1 to 2 minutes to brown the meat on that side, then turn the beef tips and brown for a minute or two on the other side. Repeat until all 4 sides are brown.
Remove the beef tips from the Instant Pot and place on a plate. Set aside until ready to add back in the Instant Pot.
Add the sliced onions, red and green bell peppers, and mushrooms (optional), to the inner liner and cook for about 3 minutes until they are soft, stirring occasionally. Then add the garlic and stir for about 1 minute. DO NOT OVERCOOK or the garlic will burn.
Press the CANCEL button on the front of the Instant Pot to stop the sauté function.Then add 2 cups beef broth and 1 Tablespoon of Worcestershire to deglaze the pan, scraping the bottom of the pan.
Add the beef tips back into the Instant Pot and stir. Close the lid on the Instant Pot. (Turn the lid clockwise until it clicks into place. The arrows should align.) Turn the Steam Release Valve on top of the Instant Pot to the Sealing position.
Press the PRESSURE COOK or MANUAL button (depends on the model) and adjust the time to 20 minutes on HIGH PRESSURE. (Note it will take time, maybe 10 minutes, for the Instant Pot to come up to pressure before it starts to pressure cook. During this time, the Instant Pot will show "On" on the display instead of the countdown of the time. After it has come up to pressure, the display will change to the timer and will countdown from the specified amount of time, 20 minutes in this case.)
After the Instant Pot has finished pressure cooking, let the Instant Pot do a NATURAL RELEASE, this means the Instant Pot releases pressure naturally, and you do nothing. After the float valve (the pin) has dropped, it is safe to open the lid.
Press CANCEL to stop the pressure cooking and then press SAUTE to cook the Beef Tips And Gravy a few minutes more to thicken the sauce.
If you need to thicken the sauce more, then mix 2 Tablespoons water with 2 Tablespoons cornstarch to create a slurry, and pour that into the Beef Tips And Gravy. Stir until the gravy is thickened. Then ladle the Beef Tips and Gravy over mashed potatoes on serving plates. Enjoy!