Mix soy sauce, sherry, mango juice, vinegar, mustard, ginger, and garlic together in a bowl. Reserve 1/3 of the mixture for a glaze. Put abalone steaks in a shallow baking dish, pour marinade over. Marinate the steaks for at least 30 minutes or refrigerate covered up to 4 hours.
Preheat an outdoor grill to high heat.
Put the reserved marinade in a saucepan, whisk in the brown sugar and cornstarch. Simmer over low heat until marinade thickens to create a glaze. Set side but keep warm.
Place 2 skewers about 2 inches apart on your work surface. Weave the marinated abalone steaks onto the skewers so that the steaks will lie flat on the grill. Alternate with fruit pieces between the steaks. Each double skewer will hold 2 to 3 steaks plus fruit.
Grill over high heat until abalone is cooked, about 1 minute per side. Brush the fruit with the glaze and serve immediately.