Thread tuna chunks onto 4 metal or soaked wooden skewers. Thread pepper and onion pieces onto 4 more skewers. Place skewers in a 13x9-in. baking dish., Whisk together marinade ingredients. Reserve half of mixture for salad dressing. Pour remaining marinade over skewers; refrigerate, covered, 30 minutes., Grill kabobs, covered, on a greased grill rack over medium heat, turning occasionally, until tuna is slightly pink in center for medium-rare (2-3 minutes per side) and vegetables are crisp-tender (10-12 minutes). Remove tuna kabobs from direct heat and keep warm while vegetables finish grilling., For salad, toss spinach, yellow pepper and cherry tomatoes with reserved dressing. For each portion, serve a tuna kabob and vegetable kabob over salad.