Turn your broiler on to high, place your top oven rack 6-8 inches below the broiler.
Place the tomatillos, jalapeños, garlic (do not peel the cloves) and half an onion on a baking sheet.
Place under the broiler for 4-5 minutes until everything starts to blister. Take the pan out, flip everything over and broil again for another 4-5 minutes.
Let everything cool to room temperature before handling.
Peel the garlic.
Remove the stems from the jalapeños, deseed only if you want the salsa less spicy, and place in the food processor along with the tomatillos, onion and garlic. Add in any juices left on the baking sheet from broiling.
Pulse the mixture a few times. Add in the lime zest and juice and the salt. Then add in the cilantro and pulse or puree until well combined but not super smooth.
Add in the sour cream and pulse again until combined. Transfer to a bowl or jar with a lid and chill completely before serving. About 4 hours.
The salsa will thicken once fully chilled.