Preheat a cast-iron grill pan to medium and brush with oil.
Place the strawberries on a large piece of foil. Top with the balsamic, orange juice, zest and rosemary. Fold up the edges of the foil to form a sealed packet and place on one corner of the grill. Cook until the strawberries are warmed through and softened, 3 to 5 minutes.
In a shallow baking dish, combine the eggs, cream, honey, vanilla and a pinch of salt. Whisk until uniformly combined. Dip both sides of the bread slices in the custard mixture, and then transfer to the grill.
Cook until the first side of the French toast is golden brown, about 2 minutes. Gently flip and cook the other side, another minute or two.
Remove from the grill and transfer to a platter. Spoon the strawberries on top and dust with powdered sugar if desired.