In a large Dutch oven, brown beef in 1 tablespoon olive oil over medium high heat.
Remove brisket to a plate. Add onions to pan and sauté until lighly browned.
Add beer and deglaze pan. Meanwhile, rub brisket on both sides with garlic and salt, then chipotles.
Remove beer and onion mixture from heat and carefully lower meat into pot and cover tightly with lid.
Place in a 350 degree oven for 3 hours.
Remove brisket from oven, then place on a cutting board to let cool. Shred meat with a fork and your fingers. Roughly chop meat and transfer back to remaining cooking liquid in pot (there won’t be much, maybe a cup – if more, pour off some).
Mix in lime juice, cilantro and scallions. Stir to combine.
Serve in warm corn tortillas topped with pico de gallo and cubed avocado.