If subbing in any grains/lentils, those should be cooked first and set aside. On stove top, heat a large skillet over medium-high heat until hot. If using an Instant Pot, press the Saute button and adjust to More/High heat. Once pan or Instant Pot insert have heated up, add the ground beef.
Season meat with salt and pepper to taste. As meat cooks it will release moisture. After about 5 minutes of cooking, drain and discard any excessive moisture . Keep cooking the beef, stirring occasionally until it's browned.
Add the diced onion, stir to mix in and saute about 2-3 minutes. add bell pepper, jalapeno pepper (if using) and minced garlic, then saute about one more minute.
Deglaze the pan by adding the cider vinegar to the pan and stir to incorporate and to help get any cooked bits off the bottom of the pan.
Add sugar, mustard, tomato paste, soy sauce and Worcestershire sauce, stir just enough to incorporate. For stove top, reduce heat to low and let mixture simmer about 10 minutes. Stirring every few minutes. For Instant Pot, close the lid and venting knob, set Manual cook for 5 minutes. After 10 minutes of natural pressure release, carefully open the valve to release remaining pressure. Carefully open the lid, give everything a stir and let it sit for another 5 minutes to allow the sauce to reduce slightly.
Toast burger buns for a few minutes in a hot skillet over medium heat before serving.