Place pork in a large non-reactive roasting pan that fits in the fridge and your oven.
To make the marinade: Place the onion, red bell pepper, scallions, crushed garlic, ground cumin, vinegar, salt and pepper in the blender or food processor. Process until well combined.
Make deep incisions on both sides of pork leg and rub the marinade all over the pork. Cover pan with plastic wrap and let marinate for 24 hours in the fridge.
Pour the beer and ground achiote over the pork leg and let it marinate for another 24 hours, turning the leg every 8 hours. When ready to cook, remove from refrigerator and let sit at room temperature for 30 minutes.
Place the oven rack in the lowest position and preheat the oven to 325F. Cover roasting pan tightly with foil and roast for about 5 to 7 hours or until tender. To keep the pork leg from drying out you will need to bathe the pork leg with the pan sauces, using a soup ladle, about every 20 minutes.
When the pork is tender remove foil from pan and let broil about 5 to 7 minutes, or until skin is crisp and crackling.