Preheat the broiler. Rinse and dry the tomatillos. Line a broiler pan with foil and arrange the tomatillos on the foil along with the poblano and serrano chill peppers. Broil until they are charred on top, about 3 minutes. Use tongs to turn and broil the other sides until charred, 3 to 4 minutes.: As the tomatillos and peppers hit intense heat you will hear a faint popping and see blistering skin, which signals caramelization. The smell becomes sharper and more roasted, adding depth that raw fruit lacks. This charring creates complex flavors that a raw salsa cannot replicate, and it also makes peeling easier for the poblano. A common mistake is crowding the pan, which results in steaming instead of charring. Make sure pieces have some space and watch closely so they do not blacken beyond a deep char, which can taste bitter.
Wrap the tomatillos and chillies in foil and let them rest for 10 minutes. Unwrap the tomatillos and chillies and peel the skin off the poblano chilli and remove the seeds. The tomatillos and serrano chilli don’t need to be peeled or seeded.: Steam trapped by the foil loosens the skin and concentrates aromas, making peeling easier and safer. After resting the poblano skin should slide off with gentle pressure, revealing smoky flesh beneath. Seeds and membranes contain most of the heat, so removing them from the poblano tames the intensity while keeping depth. A frequent oversight is skipping the rest period, which makes peeling fiddly and hotter to handle, so give it the full ten minutes for best results.
Place the tomatillos and chillies into the bowl of a food processor. Add the garlic, sugar and salt. Pulse the mixture until the ingredients are coarsely chopped.: The first pulses build texture, producing little bits of charred skin and flesh that catch the tongue. You will notice an aromatic wave of roasted notes and a faint garlic punch as the pieces break down. Coarse chopping preserves a lively mouthfeel, which I prefer to a completely smooth purée. One pitfall is over processing at this stage, which yields a too fine blend and can make the salsa thin, so stop pulsing when you see small, varied pieces.
Add 5 to 6 tablespoons of water, the onion, and cilantro. Pulse quickly until a coarse puree forms then transfer the salsa to a serving dish. Makes about 1 3/4 cup.: Adding water adjusts viscosity gently, creating a spoonable consistency without diluting flavor. The chopped onion softens while the cilantro releases fresh aromatics that lift the roasted base. When pulsing, look for a coarse, slightly chunky texture that spreads easily but still has character. If you add too much water you will lose the concentrated tang, so add measured amounts and pause to check texture. Transfer to a dish and taste for balancing, adjusting salt if needed.