To oven-steam, first fill a pan with 1" of hot water -- place the pan in the oven before adding the water to avoid sloshing it. Note: Be sure the pan is deep enough to hold about 1" of water without touching the fish. Preheat oven to 325.
Position cooling rack over a baking sheet; spray with nonstick spray. Place prepared fillets skin side down, on the rack, 1/2" apart. Season with salt and pepper.
Place the rack on top of the pan in the oven: oven steam salmon 15-18 minutes, or until desired doneness is reached. Prepare the sauce while the fish steams.
To test fillets for doneness, gently squeeze the sides -- they should spring back, not feel too soft or firm. And the color should be coral, not opaque.
In a small saucepan over medium heat, saute shallots in 1 T. buttter until soft ( do not bown). Add champagne, broth, and lemon juice; simmer until reduced by half, about 5 minute.
Reduce heat to mediumlow and stir in heavy cream. Whisk in cold butter by tablespoons, stirring constantly. Do not add more until the previous piece has melted completely.
Strain the sauce through a fine mesh strainer. Stir in the chopped chives and salt to taste. Serve immediately, or remove from heat and hold, following the instructions below.
Holding sauce -- Transfer sauce to a bowl, then nest it in a larger bowl of hot water. Cover loosely with plastic and stir often. Don't let it get too hot (it may break) or too cold (it will solidify).