Adjust oven rack to upper-middle position and set oven to broil. Line a large, rimmed baking sheet with aluminum foil.: As you start, the oven's intense heat produces rapid charring, providing audible sizzling and a hint of toasted aroma; this high heat is what creates the desired caramelized edges on the vegetables, which add essential smoky undertones to the salsa, and a common mistake to avoid is placing the rack too low where the vegetables may cook unevenly instead of blistering properly.
Place garlic, tomatillos, onion, and chiles on the baking sheet and drizzle with 1 1/2 tablespoons olive oil. Set the baking sheet on the oven rack and broil the vegetables until the skins on the tomatillos have darkened and are partially charred.: When the vegetables hit the hot foil they'll glisten and begin to steam as their skins start to ripple, and coating them with olive oil helps promote even browning and prevents sticking; if you skip coating you risk dry, uneven charring, which will result in duller flavor and a less silky final texture.
Peel the garlic and scrape all the ingredients (including vegetables, juices & oil) from the baking sheet to a blender.: You'll hear a light sizzling and see the tomatillos blister and blacken in spots, the scent shifting from grassy to slightly roasted, and it's important not to walk away because they can go from nicely charred to burnt quickly; a frequent error is overcharring everything into bitterness, so rotate the pan if some pieces are cooking faster than others.
Add cilantro, sugar, broth, lime juice, kosher salt, and black pepper. Purée to desired consistency (I prefer the salsa fairly smooth).: After roasting the garlic peels slip off easily and the pan will be fragrant with concentrated juices and oil, which you want to capture for flavor, and scraping everything preserves those caramelized bits that boost complexity; a common misstep is discarding the pan juices which leaves the salsa flatter and less aromatic.
In a medium saucepan, heat the remaining 1 1/2 tablespoons oil and pour the tomatillo salsa into the pan. Set the heat to medium and bring the salsa to a simmer. Reduce heat to low and cook until mixture reduces to about 2 cups, about 10 minutes. Season to taste with additional sugar, salt, pepper and lime juice, if needed.: Once in the blender, adding the chopped cilantro , a touch of granulated sugar , chicken broth , lime juice , kosher salt , and black pepper delivers acidity, herb, and seasoning to balance the roasted base, and you should notice a bright green flash and a citrusy scent when you pulse; be careful not to overseason early, because simmering concentrates flavors and you can always adjust later.
The salsa is even better when made the day before serving. Keep refrigerated, up to 1 week, in an airtight container.: The blender will turn the roasted mix into a glossy, green puree, releasing steam and a perfume of herb and citrus, and choosing a smoother texture makes the salsa cling well to chips or tacos; avoid overheating the blender if the mixture is still very hot, and pulse gradually to prevent splattering and an uneven texture.
In a medium saucepan heat the remaining 1 1/2 tablespoons oil and pour the tomatillo salsa into the pan: Warming a pan and adding the remaining olive oil then the salsa loosens any thick spots and helps flavors bloom while creating a gentle shimmer on the surface, and you should notice a faint gentle bubble as the mixture warms; do not pour into a cold pan as the salsa may stick or not reduce evenly.
Set the heat to medium and bring the salsa to a simmer: As the salsa reaches a simmer you'll hear a soft bubbling and see small steam wisps rising, and this stage encourages evaporation so the flavors concentrate, enhancing body and mouthfeel; avoid boiling vigorously because that can flatten delicate lime notes and make the salsa taste cooked rather than bright.
Reduce heat to low and cook until mixture reduces to about 2 cups about 10 minutes: On low heat the salsa will quietly thicken, the aroma becoming richer and the liquid developing a slightly glossy sheen, and paying attention to this reduction helps achieve a balanced intensity without being watery; don't rush by using too high heat, which can reduce unevenly and leave you with a grainy texture.
Season to taste with additional sugar salt pepper and lime juice if needed: After the reduction taste and adjust with small increments of granulated sugar , kosher salt , black pepper , and lime juice so each element sings in harmony, and you should perceive a rounder flavor and brighter finish when balanced correctly; a common error is over-salting in one go, so add cautiously and taste as you go.
The salsa is even better when made the day before serving: Resting the salsa overnight in the fridge allows aromatics to meld and the acid to soften, creating a deeper, more harmonious flavor with subtle textural changes, and storing it airtight preserves brightness; avoid leaving it at room temperature for long periods where freshness could be compromised.