1. Trim fat from meat. Cut meat into 1-inch cubes. Place meat in a medium bowl.
2. For marinade, in a small bowl stir together mustard, lemon juice, green onion, and thyme. Pour half the marinade over meat, stirring to coat. Cover and marinate in the refrigerator for 2 to 6 hours, stirring occasionally. Cover and refrigerate the remaining marinade for glaze.
3. Before grilling, place a steamer basket in a large saucepan. Add water to just below the bottom of the steamer basket. Bring to boiling. Add potatoes. Steam, covered, for 10 to 12 minutes or just until potatoes are tender. Remove steamer basket from saucepan; cool potatoes for 5 minutes. Break each onion wedge into 2 or 3 layers. Drain meat, discarding marinade.
4. On eight 6- to 8-inch metal skewers alternately thread meat, potatoes, onion, and sweet pepper, leaving 1/4 inch between pieces. Grill kabobs on the rack of an uncovered grill directly over medium coals for 12 to 14 minutes or until meat is slightly pink in the center, turning and brushing occasionally with glaze up to the last 5 minutes of grilling. (Or broil on the unheated rack of a broiler pan 3 to 4 inches from the heat for 6 to 8 minutes, turning and brushing occasionally with glaze up to the last 5 minutes of broiling.) If desired, serve kabobs with orzo.