To begin, preheat your oven to 400°F (200°C). Arrange the tomatoes, onion half, garlic cloves, and chiles on a baking sheet. Roasting these ingredients enhances their flavours for a more robust salsa.

Roast the vegetables in the preheated oven for about 10-15 minutes, turning occasionally until the tomatoes blister, the chiles blacken slightly, and the onion softens. Remove from the oven and allow them to cool for a few minutes.

Peel the garlic and remove the stems from the chiles. If you prefer a milder salsa, remove the seeds from the chiles. Transfer the roasted vegetables to a blender or food processor.

Add the chopped cilantro (if using) to the blender. Pulse the mixture a few times to achieve a chunky texture, or blend longer if you prefer a smoother salsa.

Taste the salsa and add salt and freshly ground black pepper to enhance the flavours. Start with about 1/2 teaspoon of salt and adjust as needed.

If the salsa is thicker than desired, add a tablespoon of water at a time and blend to reach your preferred consistency. Traditional salsa roja is slightly chunky but pourable.

Pour the salsa into a serving bowl. For the best flavor, let the salsa sit for 10-15 minutes to allow the flavours to meld.

Serve the salsa roja at room temperature with tortilla chips or alongside traditional Mexican dishes like tacos, enchiladas, or grilled meats. Garnish with a few extra cilantro leaves if desired.