Preheat the oven to 425 degrees F.
Score the tomatillos (i.e. cut a small slit in the bottom of each). Arrange them along with the jalapenos on a parchment-lined baking sheet. Roast in the oven, turning once 15 minutes in, until charred and soft, about 30 minutes. Set aside to cool.
Meanwhile, in a large Dutch oven or stock pot, heat 1 tablespoon of olive oil.
Add the onions to the pan and sauté, scraping up any brown bits that formed on the bottom, until translucent, 5 minutes. Add the garlic, cumin, and 1 teaspoon sea salt. Cook one minute more.
Remove the skins from the jalapenos and add them to the pot along with the tomatillos.
In a small mixing bowl, whisk together the cornstarch and ½ cup of stock until dissolved. Add this “slurry” to the pot along with the remaining stock. Bring to a boil, reduce the heat, and simmer until the sauce has thickened, about 20 minutes.
Puree the sauce with an immersion or stand blender until semi-smooth. Add the cilantro and lime juice. Taste for seasoning.