Place poblanos on a foil lined baking sheet; broil 10 min or til blackened and charred, turning occasionally. Place in a freezer bag or a bowl covered with plastic wrap. Let stand 15 minute Peel chilies; cut in half lengthwise. Discard seeds and membranes. Chop chilies and set aside.
Heat 1 t oil in large Dutch oven over med-high. Add onion; saute 10 min or til tender and golden. Spoon into large bowl.
Place flour in a shallow bowl or pie plate. Dredge beef in flour, shaking excess off. Heat remaining oil in pan over med-high. Add half of beef; sprinkle with 1/4 t salt. Cook 6 min, browning on all sides. Add browned beef to onion. Repeat browning with remaining beef and salt.
Add beer to pan, scraping up browned bits. Add chilies, 2 c water, pepper, oregano, cumin, garlic, tomatillos, and broth; bring to a simmer. Stir in beef and onions. Cover, reduce heat to med-low and simmer for 1 hour or until beef is just tender.
Stir in potatoes. Simmer, uncovered, 50 min or til beef and potato are very tender and sauce is thick, stirring occasionally. Stir in remaining 1/2 t salt and pepper. Ladle 1 1/3 cups stew into each of 8 bowls; sprinkle each serving with 1 1/2 T cheese and garnish with cilantro.