Begin by cubing the beef into 1.5-inch cubes. Try to be as uniform as possible to ensure consistent cooking of the meat. (Note - when trimming for consistently sized cubes, I trim off little pieces and freeze them to add to a stir fry later.)
Mix the rest of the marinade ingredients together in a bowl, add the beef cubes, cover, and refrigerate for 24 hours.
Prepare the vegetables by cutting into uniform 1.5-inch pieces. The more uniform all the pieces, the more consistently everything will cook. (Note – I keep all the off cuts and use them in an omelette, stir-fry, salad, or soup.)
Remove the beef from the marinade and shake off any excess marinade. Separate the beef into 4 x 6-ounce portions. Divide the peppers, onions, and mango into 4 equal portions.
To assemble each skewer, add mango, beef cube, red pepper, beef, onions, beef, green pepper, beef, yellow pepper, beef. Set aside while you preheat the grill on medium-high heat.
While the grill is heating, make the sauce. In a sauce combine the coconut milk and curry paste and cook for 5 minutes over medium heat. Add the fish sauce and brown sugar. Simmer for 10-15 minutes, stirring occasionally, while you grill the kebobs.
Add the kabobs to the grill. Cook for about 2 minutes per side until lightly charred. After the second side is cooked, lightly brush each kabob with a little oil. This will cause the grill to flare a little and give a little extra char, while flavouring the vegetables. On the final turn brush again with a little oil.
8 minutes of cooking (with the grill lid open) should have the meat at a temperature between medium rare and medium (a little pink - approximately 135°F). Size of the cubes and temperature of the grill will affect that. The beef is a tender cut, and the marinating has made it extra juicy, so cooking a little longer will still produce a tasty kabob, for those who prefer no pink.
Remove kebobs to a platter. Serve with rice, lime wedges and the warm red curry sauce. Enjoy!