Heat the olive oil in a large high-sided skillet or pan over medium-high heat. Add the onion, carrots, celery, bell peppers, mushrooms, and jalapeño pepper to the pan and cook for 5-7 minutes, stirring occasionally, until the vegetables are soft and fragrant.
Add the ground beef and garlic to the pan, and generously season with salt and black pepper, to taste. Cook for 5-6 minutes, breaking up the meat with a wooden spoon, until the beef is browned.
Add the oregano, cumin, chili powder, cayenne pepper, and tomato paste to the pan. Stir everything together and cook for another minute or two, or until the spices become fragrant.
Add the crushed tomatoes and stock/broth to the pan and bring to a boil. Once boiling, reduce to medium heat and simmer for 15-20 minutes, stirring occasionally.
Remove the chili from the heat and add extra salt and pepper if necessary.
Serve the chili immediately, topped with pickled jalapeño pepper and red onion, diced avocado, and/or cilantro, if desired. Enjoy!