How to Make It
Preheat broiler with oven rack 6 inches from heat. Place poblanos on a baking sheet and broil, turning occasionally, until lightly charred on all sides, 12 to 14 minutes. Place poblanos in a large bowl and cover tightly with plastic wrap; let stand 15 minutes. Peel away charred outer skin from poblanos and remove seeds, then slice into strips; set aside.
Whisk together flour and ¼ cup of the milk in a small bowl until smooth. Combine salt, cumin, cayenne, and remaining 1¼ cups milk in a medium saucepan. Bring to a simmer over medium-high. Whisk in flour mixture, reduce heat to medium, and simmer until slightly thickened (mixture should coat the back of a wooden spoon), 5 to 6 minutes. Remove from heat, add Monterey Jack, and stir until cheese is evenly melted.
Remove chard leaves from stems. Coarsely chop leaves and stems, keeping them separate.
Heat oil in a large skillet over high. Add onions and chard stems and cook, stirring occasionally, until softened, about 4 minutes. Add chard leaves and cook, tossing, until wilted and skillet is mostly dry, 3 to 4 minutes. Remove from heat and stir in milk mixture.
Fill tortillas with chard mixture, poblanos, Cotija, and radishes. Serve with lime wedges.