Sprinkle salmon with salt and pepper. Melt 1 tablespoon butter in medium skillet over medium-high heat.
Add salmon; sauté until just opaque in center, about 4 minutes per side.
Transfer to plate.
Melt 1/2 tablespoon butter in same skillet.
Add half of shallots and half of tarragon; sauté 30 seconds. Increase heat to high; add half of spinach and toss 30 seconds.
Add remaining spinach; toss until wilted. Divide between plates.
Melt remaining 1/2 tablespoon butter in same skillet over medium-high heat.
Add remaining shallots and tarragon; sauté 30 seconds.
Add wine and cream and boil until sauce is thick enough to coat spoon, about 3 minutes. Season with salt and pepper. Return salmon to skillet; simmer 1 minute. Arrange salmon with sauce atop spinach.