1. Line a cast-iron griddle or skillet with aluminum foil, set over high heat, and roast the tomatillos and serrano, turning often, for 5 to 7 minutes, until lightly charred and slightly softened. Transfer to a blender. Dice 1 Anaheim chile and 1 poblano chile and set aside. Add the remaining 2 Anaheims and 1 poblano to the blender and puree until smooth.
2. Press Saute-normal/medium on the Instant Pot and heat the lard. Add half of the pork in a single layer and cook, stirring occasionally, for about 5 minutes, until lightly browned. Season with 1 1/4 tsp. of the salt and 3/4 tsp. of the pepper; then, using a slotted spoon, transfer the pork to a plate. Add the remaining pork, the onion, garlic, and cumin; cook, stirring occasionally, for about 2 minutes, until lightly browned. Season with remaining 1 1/4 tsp. salt and remaining 3/4 tsp. pepper. Add flour and cook, stirring frequently, for about 2 minutes, until the flour begins to brown. Add reserved pork, the broth, tomatoes, and potatoes (if using), and scrape up any browned bits on the bottom of the pot. Press Cancel.
3. Secure the lid and set the Pressure Release to Sealing. Press Meat/Stew, and then set the cooking time for 25 minutes. When the cooking program is complete, press Cancel. Let the pressure release naturally for 20 minutes; move the Pressure Release to Venting to release any remaining steam. Open the pot and stir in the reserved diced Anaheim and poblano chiles. Taste and adjust seasoning with salt and pepper if needed.