Roast Peppers: Roast peppers on racks of gas burners over high heat, turning with tongs, until skins are blackened, 5 to 8 minutes. Transfer to a bowl and let stand, covered, until cool enough to handle. Peel peppers and discard stems and seeds. Finely chop peppers and pat dry, then season with salt and pepper.
Make sauce while peppers are standing: Finely grate 2 tsp zest from lemon and squeeze 2 tsp juice. Puree zest, juice, sour cream and 1/3 cup chopped basil in a food processor until smooth and season with salt and pepper. Transfer to a bowl and chill, covered, while preparing roulades.
Butterfly turkey: Remove skin and tender from turkey breast. Cut turkey breast in half crosswise and chill 1 piece, covered, while working with the other.
Holding a sharp knife parallel to work surface and beginning on a long side, butterfly turkey by cutting horizontally almost in half, then open it like a book.
Place between two sheets of plastic wrap and, with flat side of a meat pounder or with a rolling pin, pound turkey to slightly less than 1/4-inch thick. Butterfly and pound remaining piece.
Make roulades: Season turkey pieces well with salt and pepper and divide roasted peppers between them, spreading evenly and leaving a 1/4-inch border around edges. Top peppers with whole basil leaves. Beginning with a short side, roll up each turkey piece, gently pressing on filling while rolling (don't roll too tight or filling will slip out of the ends), and tie roulades crosswise with string. Season roulades with salt and pepper.
Prepare grill for indirect grilling. Place roulades on lightly oiled rack and grill over coals turning occasionally to sear all sides. Then move to indirect heat and grill, covered with lid, turning roulades occasionally, until meat is 170-gegrees internal, 15 to 20 minutes. Transfer to cutting board and let stand 10 minutes.
Discard string and slice roulades into 1/2-inch thick slices. Serve warm with sauce.