Heat the olive oil in a large, non-stick sauté pan over medium-high heat. Add the onion, pepper, tomatoes, and garlic and cook, stirring occasionally, until the veggies are tender, around 4-5 minutes.
Add the chicken tenders and season with salt and black pepper, to taste. Cook until the chicken develops a bit of color on each side, around 6-7 minutes.
Add the cumin, chili powder, smoked paprika, and oregano and cook, stirring occasionally, until everything is well coated and the spices are fragrant, around 2-3 minutes.
Pour in the crushed tomatoes and chicken stock. Reduce heat to medium and cover with a lid. Cook until the chicken is fully cooked through and no longer pink in the middle, around 10 to 12 minutes.
Transfer to a serving dish and top with diced avocado or pickled jalapenos and red onion, if desired, and serve immediately with cauliflower rice or your favorite side dish. Enjoy!