Heat a skillet over medium-high heat. Cook and stir chorizo in the hot skillet until brown and crumbly, about 3 minutes. Add onion and cook, stirring frequently, until onion is translucent, about 3 minutes. Transfer chorizo-onion mixture to a plate.

Pour oil into the same skillet over medium-high heat. Add frozen potatoes and seasoning salt; toss to coat potatoes in oil, then cook, stirring occasionally, until potatoes are browned and crispy, about 10 minutes.

Add diced tomatoes and chorizo-onion mixture to the skillet; stir gently to mix evenly. Cook for 4 minutes, stirring once halfway. Serve topped with avocado and cilantro.