Cut pork in 1 to 1 1/2 inch pieces and put with garlic and peppercorns in water to cover. Cook until just tender. Do not overcook as pork will continue cooking in sauce. Reserve pork stock. (A pressure cooker works well for cooking the pork.).
Roast tomatillos and chiles in roasting pan until well cooked, up to 30 minutes. In saucepan, put 1 tablespoon lard or oil. Add pumpkin seeds and cook until puffy.
Put roasted tomatillos, chiles, pumpkin seeds and 2 cups of the pork stock in blender until mixture is smooth. Don't strain.
Put remaining oil in deep pot and heat. Add tomatillo mixture, lower the heat to medium and add salt and the epazote (or oregano). If sauce is too thick, add a little more stock. Add pork and cook an additional 10 minutes. Don't worry if the sauce curdles somewhat. It should do that.
Serve with fresh corn tortillas and rice and beans as side dishes.
Note: Chicken or shrimp can be substituted for pork. Pork chops or pork loin also can be used; if so, grill or cook separately and make the sauce with chicken stock.