Sprinkle chicken legs all over with sea salt. Drizzle 1 tablespoon grapeseed oil in a roasting pan and arrange chicken legs on top. Roast chicken for 20 to 25 minutes, or until a thermometer inserted parallel to the bone reads 165° F. (Check chicken for doneness at the 20-minute mark.)
Reserve 1 pound cooked chicken for the soup: Cover and put in the fridge until needed. Set roasting pan aside to cool.
Meanwhile, set a large stockpot over medium-high heat. Add remaining 2 tablespoons grapeseed oil, and then onions. Sauté until soft, 7 minutes or so, then add all remaining ingredients but the water. Continue to sauté, stirring often, until vegetables soften and wilt, about 10 minutes.
Add roasted drumsticks, scraping any accumulated juices or brown bits in as well. Add the water. Bring mixture to a boil, then turn heat to medium-low (enough heat to produce a gently bubbling simmer). Add sea salt sparingly—just enough to bring out flavors. Cook until chicken meat falls off the bone, 2 to 3 hours.
Using a fine mesh strainer, strain mixture into jars. Set aside to cool, then seal and refrigerate for up to 5 days. Stock can also be frozen for up to 6 months.
For the tomatillo tortilla soup:
Set broiler to high.
Trim woody base from garlic cloves, then lightly smash with the side of a knife. Leave papery skin intact.
Arrange garlic, tomatillos, onion, and peppers on a rimmed baking sheet. Set under the broiler. Check every few minutes. Use tongs to set any tomatillos that have burst in a bowl, turn the peppers, and grab any garlic that's starting to blacken. Continue until all the vegetables have roasted nicely, 10 to 15 minutes total.
Set pan aside to cool. Under cold running water, remove the skin, stems, and seeds from the peppers.
Place all of the roasted vegetables in the pitcher of a blender (you may need to do this in two batches), add 2 cups chicken stock, and pulse until mixture is well blended, but still has a bit of texture. (Use caution if mixture is hot.)
Pour into a sturdy pot and set over medium-low heat. Add 2 cups remaining chicken stock, stir, and then add sea salt to taste.
Pull chicken meat from reserved roasted drumsticks, set in a small dish, and warm in the oven.
Meanwhile, set out a deep-frying pan and pour in neutral oil to 1-inch depth. Set over medium/medium-high heat. Fry tortilla strips until golden and crisp, about 3 minutes. Set cooked strips on a stack of paper towels and sprinkle with sea salt. (You can also skip the frying and use high-quality tortilla chips instead—but be mindful that store-bought chips can be very salty.)
Ladle soup into bowls, garnish with tortilla strips (or chips), chicken, avocado slices, a drizzle of crema, cilantro, and cotija. Serve soup with a lime wedge.