Cut bell peppers in half, lengthwise, and take out the seeds and the membrane out. Set aside.
Filling:
Heat up a medium cooking pan and add oil. Add diced onion and sauté until transparent. Add garlic and cook until fragrant.
Add ground beef, cover and cook for a couple of minutes. Break up all the beef clumps, mix well, and add Worcestershire sauce, ketchup, salt, and pepper. Mix well and cook until beef is almost done. Take the beef out of the pan with a slotted spoon and set side.
Gravy:
Add sliced onions to the same pan where you cooked beef and cook them until transparent. Add beef stock, Worcestershire sauce, ketchup, salt, and pepper.
In a small cup, stir cornstarch and cold beef stock together until dissolved and then, stir the mixture into the gravy. Stir everything together and cook on medium to medium-low heat for about 10 minutes.
Baking Stuffed Peppers:
Preheat oven to 375℉.
Place prepared pepper halves in a baking dish. Divide cooked ground beef among the peppers.
Spoon some gravy and onions over each stuffed pepper. Pour remaining gravy into the baking pan, around the peppers.
Bake peppers for 30 minutes. Add a slice of Pepper Jack cheese to each bell pepper and bake for a few more minutes.
Notes
Make Ahead: since the meat in the stuffing is already cooked, you can easily make these stuffed peppers ahead of time. Just prepare them up to the baking step and don't add the cheese on top. Refrigerate air-tight and when ready to bake, take off the cover, add cheese, and bake as directed. You may need to add a few extra minutes to bake time though.
Storing: you can easily store the leftovers in the same baking dish, just make sure it's cooled. Cover the baking dish air-tight and refrigerate for 3-4 days.
Reheating: you can easily reheat individual portions in the microwave, just do it in 30-45 second increments, until hot. You can also reheat the whole pan in the oven at 350℉ for 10-15 minutes.