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 half-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 tbsp chicken mixture on top.
Bake in a 450 degrees 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 tbs Cilantro-Avocado Cream and garnish with a cilantro leaf.
Serve warm.
Cilantro-Avocado Cream.
In a small bowl, mix 1/3 cup finely diced firm-ripe avocado, 2 tbsp sour cream, 1 tbsp finely chopped fresh cilantro leaves, 2 tbs lime juice, and 2 tbs milk.
Add salt to taste.
Makes about 1/2 cup.