Wash trout in cold water. Blot dry. Slash outside flesh diagonally, 2 or 3 cuts to each fish.
Cut lemons in half. Using 1/2 lemon for each trout, gently rub with cut side of lemon, squeezing as you rub. Season inside and out with salt & pepper. Sprinkle 1 tsp rosemary leaves in the cavity of each fish.
Bring wine, stock, butter, and onion to simmer in shallow skillet or roasting pan. Add fish and simmer 12-15 minutes. Cover and refrigerate, in pan, 1 hour.
Remove fish carefully from liquid. Lift off skin. Store on bed of paper towels on platter covered with foil or plastic wrap.
Refrigerate liquid separately.
Just before serving, arrange trout on beds of lettuce on individual serving plates.
Pour over each fish 1 teaspoon of poaching liquid. Serve with mustard mayonnaise.