Heat ½ tablespoon of butter and 1 tablespoon of oil in a large skillet until foamy, then add the chopped onion and saute (stirring) until tender and translucent (about 5 minutes).
Remove from the heat and transfer to a large bowl, along with the mayonnaise, dry breadcrumbs, egg, crab seasoning, Dijon mustard, salt, and sugar.
Stir to combine.
Add the crabmeat, and toss (very gently) to combine, being careful not to break up the lumps.
Divide the mixture into 4 equal portions, shaping into patties.
Coat each patty lightly in panko and place on a wire rack.
Chill (uncovered) in the fridge for 30 minutes minimum.
Heat the remaining 2 ½ tablespoons of butter and 1 tablespoon of oil together in a large skillet over medium-low heat until shimmering.
Saute the crabcakes until golden brown on the bottom (approx. 5 minutes), then flip and brown on the other side (approx. 5 minutes more).