Combine brown sugar, salt & pepper in a shallow dish.
Coat salmon filet (or filets if you've cut it into 2 pieces) on all sides with the sugar mixture.
Heat a skillet over medium-high heat with 1 tablespoon of the olive oil.
Once hot, add the filets skin side down to the pan.
Cook for about 3-4 minutes until the skin is browned and crunchy, flip and cook on all 3 remaining sides for about 3 minutes per side, until each is golden brown.
Transfer skillet to the oven and broil on high for another 3-5 minutes, until salmon is cooked through.
While salmon is in the oven, whisk together the remaining olive oil, mustard, rosemary and lemon juice in a small bowl.
Remove salmon from the oven, plate and drizzle the mustard sauce over top.