When internal temperature reaches 135, remove lamb and let rest. While resting, finish roasting potatoes at 450 or under broiler until warmed and completely browned. Heat peas on medium with a little butter (I went a little over board) then remove and mix with chopped mint.
Pour off some of the fat from drippings until you have about 4-5 tbsp of fat and all the remaining brown bits. Heat over med. Sprinkle just enough flour to thicken. You want the gravy to be runny, more like a thickened au jus. Turn temp to med high and whisk in enough stock to get the consistency you want. Heat until just bubbling and flour is cooked through. Reduce heat and season with salt and pepper.