Peel and rinse the tomatillos. Cut them in half crosswise. Place a piece of tin foil on a baking sheet or inside a large skillet. Place the tomatillos cut side down on the foil. Set the pan over a medium high heat on the stove. Cook the tomatillos over medium heat until they soften and begin to take on a nice char - turning once or twice during the process, 7-8 minutes. Set aside.
If you have a gas cooktop or grill, turn the flame onto medium high heat and rest the jalapeño and poblano peppers over the flame until it blisters and blackens, using your tongs to turn and rotate several times until the entire pepper is blackened. (I rest the peppers directly on the iron grate over the flame, so the flames lap at the skin.)
If you don't have a gas stovetop or grill, preheat the oven to 400 degrees. Place peppers on a baking sheet. Roast for 20-25 minutes until softened and slightly blackened.
After peppers are cooked place them in a glass bowl and cover the bowl with plastic wrap. Set aside to cool. When peppers are cool enough to handle, slough off the charred skin with your fingers and discard (it should come away easily).
Cut off the stem of the peppers. With a knife, cut the peppers in half, lengthwise. Remove the seeds and discard. Cut the peppers into large chunks and place into a small mini-prep food processor. Add the tomatillos, garlic and cilantro.
Using the chop or pulse button on the mini-prep, process the mixture until it's the consistency of a chunky salsa.
Stir in the white onion.
Season to taste with kosher salt and lime juice. Serve with tortilla chips, burritos, tacos, grilled steaks, chicken, pork or fish.