Grill or sauté the chicken until it takes on some color, don't over cook. Cut it into 1/4- x 2-inch strips and reserve. Grill the onions (or sauté in a very hot pan) until they are tender and blackened in spots. Add them to the chicken.

In a heavy skillet over medium-high heat, cook the tomatoes without any oil until blackened in spots all over and roughly chop. Add the tomatoes to a saucepan along with the stock, garlic, serranos and cilantro. Simmer over low heat for 30 minutes. Remove the cilantro.

Add the poblano chile, chicken and onion to the broth and heat through, about 5 minutes. Off the heat, stir in the lime juice. Ladle into soup bowls and sprinkle a pinch of oregano over each bowl, along with some tortilla strips (if using) and a lime wedge.