Place poblano chiles on a foil-lined baking sheet; broil 10 minutes or until blackened and charred, turning chiles occasionally. Place in a heavy-duty zip-top plastic bag; seal. Let stand 15 minutes. Peel chiles; cut in half lengthwise. Discard seeds and membranes. Chop chiles; set aside.
Heat 1 teaspoon oil in a large Dutch oven over medium-high heat. Add onion; sauté 10 minutes or until onion is tender and golden brown. Spoon onion into a large bowl.
Place flour in a shallow bowl or pie plate. Dredge beef in flour, shaking off excess. Heat remaining 2 teaspoons oil in pan over medium-high heat. Add half of beef mixture; sprinkle with 1/4 teaspoon salt. Cook 6 minutes, browning on all sides. Add browned beef to onion. Repeat procedure with remaining beef mixture and 1/4 teaspoon salt.
Add beer to pan, scraping pan to loosen browned bits. Add chopped chiles, 2 cups water, pepper, oregano, cumin, garlic, tomatillos, and beef broth; bring to a simmer. Stir in the beef mixture. Cover, reduce heat to medium-low, and simmer for 1 hour or until beef is just tender.
Stir in potato. Simmer, uncovered, 50 minutes or until beef and potato are very tender and sauce is thick, stirring occasionally. Stir in remaining 1/2 teaspoon salt and black pepper. Ladle 1 1/3 cups stew into each of 8 bowls; sprinkle each serving with 1 1/2 tablespoons cheese. Garnish with cilantro, if desired.