Pat the salmon fillets dry with paper towels and season generously with salt. Add 1 tbsp of olive oil to a large skillet and set over medium heat. Once shimmering, add the salmon and sear on one side for about 5 minutes.
Flip and sear on the other side until the fish registers 50°C (125°F) when an instant-read thermometer is inserted into the thickest part of the fillet - about 5 more minutes. Remove from the pan and set aside.
Add the butter to the pan in which the salmon was cooked. Once melted, add the mushrooms and a generous pinch of salt. Cook, stirring every so often, until the mushrooms release their moisture, reduce in size considerably and begin to brown - about 10 minutes.
Add the shallots, garlic and miso paste and cook for another 2 minutes, just until the shallots are softened. Add the lemon zest and cook until fragrant, only another 30 seconds.
Sprinkle over the flour and cook for a minute or two, stirring it into the mushrooms. Then, pour over the chicken (or veggie) stock and bring to a simmer.
Allow to simmer for 3-5 minutes, just until the sauce is thickened and lightly viscous. Stir in the creme fraiche. Off heat, stir in 1 tsp of lemon juice. Taste to adjust for seasoning, adding more lemon juice, salt or pepper where needed. Spoon the sauce over the cooked salmon fillets and serve immediately.