Preheat a comal or small saute pan over medium-low heat. Once hot, toast chiles de árbol for about a minute until toasted and fragrant. Remove from the heat.
Place the tomatoes, tomatillos, jalapeños, and garlic in a medium saucepan. Cover with water and set over medium-high heat, simmer for about 10 minutes until the tomatoes and tomatillos are cooked and mushy and the jalapeños have softened and changed their color.
Using a slotted spoon, transfer the tomatoes, tomatillos, jalapeños, and garlic to blender. Incorporate the chiles de árbol, onion, cilantro, cumin, and salt and puree until smooth.
Set a medium saucepan over medium heat, once hot add the oil. Once the oil is hot, pour in the salsa and be ready to partially cover with the lid as it will jump. Cook for 4 to 5 minutes, until thickened and seasoned and color has deepened.