Start by adding the mustard, honey, and garlic to a small mixing bowl. Mix really well and set it to the side.
Next, heat a large skillet on medium heat. Let it heat until it begins to lightly smoke.
Add in the oil, give it a swirl, and then add in the salmon fillet, skin side down. Season with salt.
Let this cook for about 4 minutes, then with a large spatula, carefully flip it over. The skin should be nice and crisp.
Next, start brushing the mustard and honey mixture on top of the salmon. Continue to brush for the next few minutes. Add in the pad of butter, and swirl it in the skillet. Cook until the salmon is fully cooked, about 10 minutes total.
Remove it from the skillet, and pour over any additional sauce.