Remove husks from tomatillos and cut lengthwise.
Cut poblano away from core and into roughly four flat strips.
Heat nonstick skillet over medium-high heat.
Place tomatillos cut-side down on hot skillet, and add poblano strips, whole jalapeno, and garlic to skillet. Allow to cook undisturbed until the tomatillos soften slightly and a nice char develops. Using tongs, flip ingredients over and allow all sides to develop some char. When complete, move skillet away from heat and allow it to cool down while you prepare remaining ingredients (onion, cilantro, lime juice).
Remove jalapeno from skillet, chop in half, and add to food processor. Set aside remaining half (to add additional heat to taste)
Add tomatillos and poblano to food processor.
Pulse until mixture has a fine texture, but not totally smooth.
Add lime juice and cilantro, and processor until cilantro is incorporated.
Taste and add additional jalapeno for spiciness, additional lime juice, and/or salt.
In serving or storage dish, combine poblano mixture from food processor with diced onion and mix with a spoon until onion is distributed throughout.