Preparation

The night before

• Make the sauce
  Put the tomatillos and the jalapeño in a 4-quart saucepan and add water to within 1 inch of the top. Bring to a boil, then lower the heat and simmer, uncovered, turning the tomatillos occasionally, until they’re khakigreen all over and very tender, about 15 minutes. Using a slotted spoon, transfer the tomatillos to a blender. Halve the jalapeño, and add one half to the blender. Add the cilantro, garlic, cumin, and 2 tsp. salt and purée (you should still see the tomatillo seeds). Taste and blend in the other jalapeño half if you prefer a spicier sauce. Add the cream and pulse to blend. Transfer to a bowl and let cool to room temperature.
• Assemble the Enchiladas
  Oil a 9×13–inch baking dish and set aside.
• Add the potato and cook until just tender, about 4 minutes. Drain and set aside.
• Meanwhile, cook the chorizo in an 11- to 12-inch nonstick skillet over medium-high heat, breaking up the meat, until cooked through, about 4 minutes. Use a slotted spoon to transfer the chorizo to a large bowl. Discard all but 1 Tbs. fat from the pan. (If there’s less than 1 Tbs. of fat, add a bit of the vegetable oil.)
• Set the skillet over medium heat. Add the bell pepper and onion and cook, stirring occasionally, until softened, about 5 minutes. Stir in the potato and then cook undisturbed until browned on the bottom, 3 to 4 minutes. Add the mixture to the chorizo. Add the green chiles, if using.
• Whisk the eggs, 1 tsp. salt, and several grinds of black pepper in a medium bowl. Melt the butter in the skillet over low heat and cook until the foam subsides. Add the eggs and cook without stirring until they begin to set on the bottom, about 20 seconds. Draw a wooden spoon across the pan a few times to form large curds. Continue to cook, stirring occasionally, until the eggs are barely set and still quite moist, 1 to 2 minutes—do not cook through. Add the eggs to the chorizo mixture.
• Stir 1 cup of the tomatillo sauce into the eggchorizo mixture. Cover the remaining sauce and refrigerate.
• Brush both sides of one tortilla with oil and put on a microwaveable plate. Oil the remaining tortillas on one side and stack, oiled side up. Cover with a paper towel and microwave the tortillas until warm and pliable, 1 to 1-1/2 minutes. Arrange about 1/3 cup of the egg mixture in a line down the center of each tortilla and roll it up. Place each filled tortilla seam side down in the prepared baking dish. Cover the dish with plastic wrap and refrigerate overnight.

In the morning

• Position a rack in the center of the oven and heat the oven to 350°F.
• Uncover the enchiladas, pour the remaining tomatillo sauce evenly over them, and top with the cheese. Bake until the sauce is bubbly and the cheese is browned, 30 to 40 minutes. Let stand for 5 minutes and then serve with the sour cream, if using.