Blanch the tomatillos in 1 quart boiling water for 3 minutes. Drain and save 1/2 cup of the cooking water. Set aside, cool, and remove husks. Place tomatillos in a blender with the garlic, salt, and serrano chiles. Add 3 to 4 tablespoons cooking liquid and blend on high speed for 10 seconds. Set sauce aside.
Heat 1 tablespoon olive oil in a large skillet over medium-high heat. Add the Mexican sausage and fry until browned and completely cooked through. Transfer sausage to paper towels to drain and set aside.
Simmer sauce for 5 minutes in a pan wide enough to dip tortillas. Heat the remaining oil in a large skillet over high flame. With tongs, dip 1 tortilla at a time in the hot oil for a few seconds to coat and heat and then dip the tortilla in tomatillo sauce. Place on a serving plate and fill with 1/12th of the Mexican sausage, 1 tablespoon minced onion, and 1/2 teaspoon minced cilantro. Roll the tortilla into the shape of an enchilada. Repeat with remaining tortillas. Top with sauce, sour cream, strips of green chile, and a cilantro sprig. Serve immediately.