Heat a skillet over medium-high heat. Add chorizo, breaking it apart until crumbly. Saute until it starts to brown, about 3 minutes. Add chopped onion and cook, stirring frequently, until onion is translucent, about 3 minutes. Transfer chorizo-onion mixture to a plate.
Pour oil into pan and heat over medium-high heat. Add frozen hash brown potatoes and seasoning salt. Stir to coat potatoes in the oil.
Let the potatoes brown on one side, then flip once after about 5 minutes. Both sides should be browned and crispy, about 5 minutes per side. Add the diced tomatoes and the chorizo and onion mixture; stir gently to mix evenly. Cook for 4 more minutes, stirring once to rotate the ingredients from the bottom of the pan to the top for even cooking.