Pat salmon dry and season liberally on both sides with salt and freshly cracked pepper.
Preheat cast-iron skillet over medium-high heat. Add olive oil and allow oil to heat fully.
Place salmon filets, skin side up, in hot oil. Allow to cook for about 3 min or flesh is crispy and a deep golden brown.
In the meantime, whisk together garlic, dijon mustard, maple syrup, water, and red pepper flakes.
Flip salmon and sauce to pan. Add butter, and baste salmon with a spoon another 3-4 minutes or until salmon flakes easily with a fork. Serve warm with sauce spooned over salmon.