In a small saucepan over medium heat, bring the chicken stock and the Green Chile Sauce to a simmer. Hold warm.
In a large, enameled cast-iron Dutch oven over medium-high heat, add the oil. When hot, add the onions and serrano chiles and saute until the onion is just starting to color, 4 to 5 minutes.
Add the rice and stir to coat, Cook for 3 to 4 minutes, stirring constantly, to toast the rice a bit. Add the garlic, salt and pepper and stir to combine.
Ladle in the chicken stock and green chile mix, about 1 cup at a time, stirring and allowing the rice to absorb the liquid. Do this 3 to 4 times, and then add the rest of the liquid. Stir, cover tightly and cook for 12 to 16 minutes, stirring twice during this time.
Add the lime juice and cilantro and stir to combine. Place in serving bowl and garnish with the pepitas.
Green Chile Sauce:
In a small Dutch oven over medium-high heat, add the oil. When hot, add the onions and jalapeno and saute until just starting to caramelize, 4 to 5 minutes. Add the garlic and cook 1 minute more.
Reduce the heat to medium, sprinkle the onion mixture with the flour and stir constantly until lightly browned, about 2 minutes. Add the reserved liquid from the chiles, 1 cup water, the vinegar and salt and stir until the liquid is smooth and all the flour is dissolved.
Add the chiles and pepper and simmer for 5 to 6 minutes. Puree in a blender, or with a stick blender, and add the lime juice. Adjust to taste for salt and pepper. Refrigerate for up to 1 week. Yield: 4 1/2 cups