In a 2 1/2- to 3-quart pan over medium-high heat, frequently stir onion in olive oil until onion begins to brown, 4 to 7 minutes. Add cumin seeds and oregano; stir until fragrant, about 30 seconds. Add chilies, tomato paste, vinegar, and 1/2 cup water; bring to a boil, then reduce heat and simmer gently, stirring often, to blend flavors, about 5 minutes. Add chicken and stir until hot.
Meanwhile, arrange tortilla chips in a single layer in a 12- by 17-inch pan. Sprinkle equal portions of cheese on each chip and spoon about 1 tablespoon chicken mixture on top.
Bake in a 450° regular or convection oven until cheese begins to bubble, about 3 minutes. With a spatula, carefully transfer tortilla chips to a platter. Top each with 1 teaspoon Cilantro-Avocado Crema and garnish with a cilantro leaf. Serve warm.
Cilantro-Avocado Crema. In a small bowl, mix 1/3 cup finely diced firm-ripe avocado, 2 tablespoons sour cream, 1 tablespoon finely chopped fresh cilantro leaves, 2 teaspoons lime juice, and 2 teaspoons milk. Add salt to taste. Makes about 1/2 cup.