Rub lamb with olive oil. Generously salt (gros sel) the lamb. Mix the peppercorns together and pat on lamb. (This should be done at least 4 hours in advance, preferably overnight).
In a large bowl, mix together potatoes, onions, carrots, garlic and thyme. Add oil, season and mix well.
When pan is hot, pull out and dump veggies into the pan. There should be a good sizzle. Mix the veggies around and roast for 10 minutes. Mix again and place lamb on top. Place in oven and roast until brown, about 20 to 30 minutes.
Turn oven down to 375 degrees, cover the lamb with foil to prevent burning and roast another 20 to 30 minutes. I shoot for medium rare, about 130 degrees. Mix veggies a couple of times during the cooking.
Place lamb on a cutting board and let rest 10 minutes. Make sure the veggies are fully cooked and check for seasoning. If not, place back in oven another 10 minutes.
Serrano Chile-Mint Jelly: In a non-reactive pan, dissolve sugar in juice and water. Pour 2 tablespoons of the warm liquid in a small saucepan and add gelatin. Dissolve on low heat and set aside. Raise to medium-high under the first saucepan and add the chiles, mint, zest and salt and bring to a boil. Reduce heat to low and cook for 10 minutes, occasionally stirring. Remove from heat and add dissolved gelatin. Store in refrigerator overnight, stirring occasionally to break up the gelatin. It will be a soft jello-like consistency.