Place a comal or a cast iron skillet over medium heat and allow to heat up until smoky.
Place tomatillos, allow to roast for about 10 minutes or until they soften and charred spots appears, make sure to flip them from time to time to roast evenly.
Half way from tomatillos being roasted, add onions and garlic and roast until onions are translucent.
Add chilies and roast those for a few seconds too (read notes).
Peel garlic and add it to a blender with chilies, onions, and half of tomatillos.
Add 1 teaspoon salt and ½ cup of water. Blend at high until you'll have a smooth salsa.
Add the remaining tomatillos. Pulse a few times to blend and combine the ingredients.
Adjust salt to taste and transfer the salsa to a bowl or jar.
Pay attention when roasting chilies, as chile de árbol burns easily, so make sure you flip them almost immediately, they can be done in just a few seconds, same as guajillo.
I suggest to roasting garlic with the skin on, I find that garlic tends to get a burnt flavor when roasted without.
If you adding a bottle of tomatillo salsa instead of fresh tomatillos, make sure you reduce the amount of chilies, as the store-bought salsa is already spicy.
You can also use a molcajete or mortar to make this delicious tomatillo red chili salsa. Just grind first the chilies to pulverize them, then proceed with garlic, onions, and grind last tomatillos. This is actually the most traditional way of making tomatillo red chili salsa in Mexico.