In a large skillet over medium-high heat, add oil. Onion and pepper and cook, stirring, till starting to brown. Add chili powder, cumin, salt, and pepper and toss, continuing to cook 3 to 5 minutes. Remove vegetables from pan and set aside.
Return skillet to medium-high heat and add halloumi. Cook 2 to 3 minutes each side till golden brown.
Add vegetable mixture back to the pan and toss everything together.
Serve with tortillas, cilantro, spinach, sour cream, and avocado.