Heat oil in a large skillet and cook chopped onion until it is transparent. Add garlic and continue cooking until garlic turns light brown. Add tomatillos, tequila, chilies, salt, and sugar. Cook over medium high heat to reduce the liquid and cook tomatillos. Let cool, pour into blender and blend into smooth paste.
Cut lamb to an even thickness of 2 to 2 1/2 inches, sprinkle with salt and ground black pepper. Rub half the paste into the lamb. Marinate the lamb up to 1 day in the refrigerator. Bring the lamb to room temperature before grilling.
Mix tomato paste and the brown sugar well and set aside.
When ready to grill, set grids at the medium setting. Preheat grill to high. Cook lamb to desired degree of doneness (160°F for medium and 170°F for well done) as measured with meat thermometer. Remove the lamb from grill, slice and serve with salad, vegetables, and dry red wine.
