Season the salmon with salt and pepper. Heat a tablespoon of butter in a skillet over medium-high heat, then sear the salmon for 3–4 minutes per side until just cooked through. Remove and set aside.
In the same pan, add the remaining butter. Sauté the shallot for 1–2 minutes until translucent, then stir in the garlic for about 30 seconds until fragrant.
Add the sliced mushrooms. Cook 4–5 minutes, stirring occasionally, until they release their moisture and start to brown.
Pour in the broth and bring to a simmer. Stir in the heavy cream and Dijon mustard (if using). Let it simmer a couple of minutes until slightly thickened.
Finish with lemon juice, and season with salt and pepper to taste. Return the salmon to the pan to warm through for 1–2 minutes.
Sprinkle with chopped parsley or chives and serve hot over or beside the salmon. Voilà—restaurant vibes at home.
