Melt 1 T butter in medium skillet over medium-high heat.
(If you use a large skillet, you'll need to use more butter. For a better finished result, medium is preferred.) Add salmon; saute' until just opaque in the center, about four minutes per side if thick.
Transfer salmon to a plate.
Melt 1/2 T butter in same skillet.
Add 1/2 the shallots and half the tarragon; saute' 30 seconds.
Increase the heat to high; add half the spinach and toss 30 seconds.
Add remaining spinach and toss until wilted.
Divide the spinach between two plates.
Melt remaining 1/2 T butter in same skillet over medium-high heat.
Add remaining shallots and tarragon; saute' 30 seconds.
Add wine first and then cream and boil until the sauce is thick enough to coat a spoon, about 3 minutes (or less if using larger skillet).