How to Make It
Preheat broiler with oven rack in middle of oven. Place poblanos on a baking sheet, and broil, turning occasionally, until blackened all over, 10 to 15 minutes. Transfer poblanos to a medium bowl, and cover tightly with plastic wrap. Let steam 15 minutes. Peel and discard skin from poblanos. Remove and discard stems and seeds. Cut poblanos into 2- x 1/2-inch strips, and set aside.
Heat 3 tablespoons oil in a large skillet over medium-low. Add onion, garlic, and tied epazote stems; cook, stirring often, until onion is just starting to soften, about 4 minutes. Remove and discard epazote stems. Add squash and salt; increase heat to medium. Cover and cook, stirring occasionally, until squash is just tender, 8 to 10 minutes.
Stir in poblano strips, squash blossom pieces, and 1/4 cup loosely packed epazote leaves. (Reserve remaining epazote leaves for another use.) Sprinkle with queso fresco. Cover and cook over medium until cheese is just beginning to soften, about 1 minute.
Transfer mixture to a serving platter. Drizzle with red wine vinegar and remaining 2 tablespoons oil. Season with salt to taste. Serve immediately.
Epazote can be purchased from Latin markets.