Heat a large skillet over medium-high heat and add olive oil. Season salmon with salt and pepper, then place in the skillet (skin side up, if the salmon has skin) and cook until opague in the center and golden on each side, about 5-6 minutes for salmon that is 1-inch thick. If you use salmon with skin, simply cook it skin side up the entire time. Remove salmon and set aside, then add remaining 1/2 tablespoon of butter, shallot, garlic, and sage. Stir well to coat then cook for 1-2 minutes until sizzling, then add in white. Cook for 2-3 minutes, allowing it to bubble and slightly reduce, then whisk in milk and mustard. Continue to whisk and cook while milk bubbles on the sides and thickens, stirring for a minute or two. Taste and season additionally or whisk in a bit more dijon if desired.