Combine first 6 ingredients; stir well. Spread mustard mixture over meaty portion of lamb racks; set aside.
Process pecans in a food processor until ground. Combine pecans, 1/4 teaspoon salt, pepper, butter, and 1 tablespoon lemon juice. Pat pecan mixture over mustard-coated lamb.
Place roasts on a rack in a roasting pan, fat side out and ribs crisscrossed. Insert meat thermometer into thickest part of lamb, making sure it does not touch bone. Roast at 425° for 35 to 40 minutes or until thermometer registers 150° (medium-rare). (Cover roasts loosely with foil after 25 minutes, if necessary, to prevent excessive browning.) Let stand 10 minutes before slicing.
Lamb know-how: When you're preparing racks of lamb, let a butcher do some of the work for you. Ask him to french the chine bones (remove fat and gristle) and to trim the cap fat, leaving only the tender eye meat and resulting in a beautiful entrée presentation.