DO-AHEAD TIP: If using bamboo skewers, presoak them in water for 30 minutes, to prevent burning on the grill. Marinate beef skewers for 30 minutes (see recipe).
Preheat grill to MEDIUM.
Place beef cubes in a large zipper-topped plastic bag. In a small bowl, combine next 9 ingredients (coconut milk through red onion). Pour mixture over beef cubes and shake well to coat.
Alternately thread beef cubes and bell pepper squares onto skewers then carefully place them in the bag. Seal and refrigerate for 30 minutes. Remove kabobs from bag, discarding marinade. Add mushrooms to the ends of the skewers. Lightly oil the grill grate.
Grill kabobs over indirect heat for 15 to 20 minutes, turning once half-way through cooking time, until cooked to desired level of doneness. Watch carefully to prevent burning! Serve over fresh spinach.
Enjoy!!