In a large skillet, heat 2 tablespoons olive oil over medium heat and sear beef 3 to 4 minutes per side until golden.
In a large slow cooker, add beef, onions, garlic, chipotle peppers, thyme, beer and sea salt and freshly ground pepper to taste and cook on low for 8 hours until beef is cooked through.
In a blender or food processor, add tomatillos, serrano chili, cilantro, lime juice and sea salt and freshly ground pepper to taste. Blend 1 minute until smooth. Place in a small bowl and add the chopped avocado.
Place beef on warm tortilla and top with salsa Verde