Tomatillos look like small green tomatoes with papery husks. They can be found in Latin American grocery stores and sometimes in supermarkets.
Place tomatillos and 2 tablespoons lime juice in a blender or the bowl of a food processor. Puree until smooth, then transfer mixture to a medium bowl.
Cut each avocado lengthwise around pit and twist halves apart; remove pit. Peel each avocado half and cut into small dice. Add to tomatillo mixture along with scallions, poblano, jalapeno, garlic, and cilantro. Mix well and season to taste with salt, pepper, and lime juice. Let sit, covered, for 1 hour at room temperature before serving. Serve with grilled meat, fish, or corn tortilla chips.