Preheat the grill to 400 degrees F (205 degrees C).
Flake the crabmeat into a large bowl. Add the breadcrumbs, mustard, mayonnaise, hot sauce, Cajun seasoning, egg, and onions. Mix well, but be careful not to overwork the cakes.
Divide the mixture into 8 balls. Flatten each one into a cake.
Lightly brush the cakes with the oil.
Place the cakes on the preheated grill and cook for 3 minutes. Carefully lift and twist each cake to get a diamond-shaped sear. Cook for 2 additional minutes.
Flip the cakes and cook for 4 to 5 minutes on the other side.
Remove from the grill to a serving platter and serve immediately.