Step 1

In a medium saucepan add the tomatoes and tomatillos and cover with water. Bring to a boil, then reduce to a simmer, and cook until soft, about 10 minutes.

Step 2

Place the cooked tomatoes and tomatillos in a blender along with the chiles de árbol, oregano, onion slice, and salt, and puree until smooth. Adjust salt if necessary.