Preheat oven to 400° Fahrenheit and line a large sheet pan with parchment paper.
Mix together the Mexican seasoning ingredients in a small bowl or ramekin.
Wash and dry potatoes. Chop potatoes into small cubes and place them in a bowl. Chop onion and bell pepper as noted (if using) and add to the bowl.
Drizzle avocado oil over the potatoes and toss to coat. Sprinkle the seasoning mixture over the potatoes and toss again to coat evenly with the seasoning.
Spread the potatoes out evenly on the sheet pan. Place in the oven to bake for 30-40 minutes, or until the potatoes are soft when pierced with a fork.