If using fresh chorizo, slowly render the fat in a skillet over low heat. Gradually begin turning up the heat, allowing all sides of the chorizo to cook and eventually brown. Periodically deglaze the pan with water and scrape up the brown bits. This will prevent the bottom of the pan from burning and it will incorporate the yummy caramelized bits back onto the sausage. This should take approximately 20 minutes, give or take.
Allow the sausage to cool for a bit and then slice each link into 10-12 pieces.
Briefly saute the chopped onions in the leftover chorizo fat (you can also use olive oil if pre-cooked chorizo was used).
Preheat the oven to 300 degrees F.
Ladle 1/4 cup of tomato sauce into the bottom of mini cast iron skillets or ramekins.
Drizzle some Sriracha on top of the tomato sauce (the amount will depend on your preference for heat. I used about 2 tablespoons per pan.)
Line the chorizo slices in the bottom of each skillet and top with another dollop of tomato sauce (approximately 2 tablespoons per pan, doesn't need to be exact.
Divide the onions in half and evenly spread over the chorizo.
Layer on the cherry tomatoes followed by the arugula.
Break the eggs into separate ramekins, leaving the yolk intact, and then carefully pour the into each skillet.
Bake until the egg whites are just set, 10-15 minutes (they will continue to cook outside of the oven).
Sprinkle salt and fresh ground pepper on top before serving