Line a heavy grill pan with aluminium foil and place over a high heat. Arrange the tomatillos, chillies and garlic in a single layer. Cook until soft and darkened (or blackened, for a smokier flavour), turning as necessary. The chillies and garlic will finish before the tomatillos so remove them as they are done.
Peel the garlic and place in a mortar and pestle with salt to taste (salt generously). Grind to a smooth paste then add the chillies and grind until smooth. Transfer to a bowl.
Add as many tomatillos as you can fit in your mortar then use the pestle to crush them, leaving them slightly chunky. Work in batches as needed.
Combine the crushed tomatillos with the chilli and garlic paste then serve.