Heat 3 tablespoons oil in large skillet over high heat and cook steak, in batches, turning once, until desired doneness, about 1 minute. (Be sure not to overcook so steak stays juicy.) Remove steak and set aside.
Heat remaining 1 tablespoon oil in same skillet over medium-high heat and cook onions, stirring occasionally, until very soft and golden, about 8 minutes. Stir in poblano pepper and garlic and cook about 1 minute.
Stir in water, cilantro and remaining 1/2 tablespoon Bouillon and cook, about 1 minute. Remove from heat and set aside.
Slice steak. Heat 1 tortilla in large nonstick skillet over medium-high heat 1 minute. Place 1/4 of the sliced steak, then 1/4 of the onion mixture and 1/4 of the cheese on one half of the tortilla. Fold the other half of the tortilla over the filling.
Cook over medium-high heat, pressing down with a spatula, until bottom is lightly browned. Carefully turn quesadilla and cook until lightly browned and cheese is melted, about 2 minutes. Remove quesadilla and keep warm. Repeat with remaining ingredients.
To serve, cut each quesadilla into wedges.
Steak can be enjoyed in countless ways. Follow these tips to cook tender, juicy steak you can’t resist.