1. Put the morels in a bowl and pour hot water over them. Place small lid over the morels to keep them submerged and let stand until softened, about 20 minutes.
2. Lift the morels from the soaking liquid and rinse them of any grit. Reserve the liquid. Roughly chop any large morels.
3. Melt 2 Tablespoons of butter in a skillet. Add the cremini mushrooms, season with salt and white pepper and cook over moderate heat until browned, about 5 minutes.
4. Add the morels and cook, stirring, until they start to darken, about 3 minutes. Add wine and simmer over moderately high heat until reduced, about 3 minutes.
5. Add the stock and slowly pour in the reserved morel soaking liquid. Simmer until reduced by two-thirds, about 6 minutes. Add the cream and tarragon and simmer until thickened, about 4 minutes. Add the lemon juice and season the sauce with salt and pepper.
6. In a large, nonstick skillet, melt the remaining 2 Tablespoons. Season the salmon fillets with salt and white pepper and add them to the skillet, skin side down.
7. Cook over high heat for 5 minutes then reduce heat to medium and cook until the skin is browned and very crisp, about 12 minutes longer.