Place a cast iron skillet or comal over medium-high heat for 2 to 3 minutes until hot.

Char the Roma tomatoes on the hot skillet for 2 to 3 minutes per side until blistered and dark spots form, about 8 to 10 minutes total.

Char the quartered white onion on each side for 2 to 3 minutes until dark brown spots form.

Add the serrano peppers and garlic clove to the skillet and char for about 2 minutes, turning occasionally.

If this is the first use, grind a small handful of sea salt in the molcajete for 30 seconds to season, then wipe it clean.

Add the fresh cilantro to the molcajete and grind in circular motions with the tejolote for about 1 minute until fragrant.

Add a generous pinch of salt and the charred garlic clove to the molcajete and mash into a rough paste.

Add the charred serrano peppers and grind for 1 to 2 minutes until mostly broken down but still slightly chunky.

Add the charred onion to the molcajete and grind for 1 to 2 minutes, maintaining some texture.

Add the charred tomatoes and grind for 2 to 3 minutes until mostly broken down with small chunks remaining.

Taste the salsa, adjust salt as needed, and stir in lime juice from half a lime if desired.