• Scrub or peel the sweet potato. Trim the ends and cut the potato lengthwise into ½-inch-wide slices, then crosswise into ½-inch cubes.
• Peel and coarsely chop the red onion.
• Remove the husks from the tomatillos; rinse the fruit and pat dry, then coarsely chop.
• Trim the root ends from the scallions; thinly slice the scallions on the diagonal and set aside one-fourth for garnish.

• Pat the pork dry with a paper towel; season with salt and pepper.

In a large frying pan over medium-high heat, warm 2 tablespoons oil until hot but not smoking. Add the pork and cook, turning frequently, until browned but still faintly pink within, 8 to 10 minutes. Transfer to a plate. Do not clean the pan.
While the pork cooks, prepare the garlic and serrano.

• Finely chop, press, or grate enough garlic to measure 1 teaspoon.
• If using the serrano, remove the stem, ribs, and seeds; finely chop the serrano. Wash your hands after handling.

In the same pan used for the pork, if dry, add 1 to 2 tablespoons oil. Warm over medium heat until hot but not smoking. Add the sweet potato, onion, scallions, garlic, oregano, and as much serrano as you like. Season with salt and pepper and cook, stirring occasionally, until the vegetables start to soften, 5 to 7 minutes. Add the tomatillos, green chiles, ¾ cup water, and as many chile flakes as you like. Bring to a boil, reduce to a simmer, and cook, stirring occasionally, until the sweet potato is tender and the sauce is slightly reduced, 5 to 7 minutes. Season to taste with salt and pepper.
When the vegetables are almost done, chop the cilantro.

• Coarsely chop the cilantro.

Transfer the vegetables and sauce to individual bowls. Top with the pork, garnish with the cilantro, pumpkin seeds, and remaining scallions, and serve.