Start by heating a heavy cast-iron skillet or griddle over medium-high heat. Once it's got a little heat going, place your tomatillos in the pan, cut side down. Let them roast until the bottom side is nicely charred—a process that takes about 10 minutes.

You'll notice their pale green skins fading into a deeper olive color. Carefully flip them over and repeat on the other side for another 10 minutes. Be cautious; you don’t want them to burst. Once charred, transfer the tomatillos to a bowl and let them cool for a bit.

Now, let’s amp up the flavor. In the same skillet, toss in the serrano chiles, unpeeled garlic clove, and the slice of onion. Toast these for a few minutes, turning them often. You want the chiles to get a little charring, and the garlic to soften until it’s slightly darkened but not burnt.

The onion should be lightly colored without turning black. Remove from heat once they are done.

Here’s the fun part! Peel the garlic (it should be super easy after roasting) and add it to a blender along with the toasted onion and chiles. Don't forget to include the cooled tomatillos and any juices that accumulated in the bowl.

Now, add the avocado, cilantro, lime juice, and salt to taste. Blend everything until you reach a smooth consistency, and give it a taste. Adjust the seasonings if needed, and voilà—your vibrant Avocado Tomatillo Salsa is ready to serve!