In a heavy saucepan boil vinegar, water, onions, sugar, and 2 teaspoons chopped rosemary, stirring occasionally, 7 minutes, or until onions are slightly softened.
Remove pan from heat and let stand 15 minutes.
In a coarse sieve set over another saucepan drain onions, reserving cooking liquid.
Transfer onion compote to a bowl and cool.
Boil reserved cooking liquid with remaining 2 teaspoons chopped rosemary until reduced to about 3/4 cup.
Add broth and boil liquid until reduced to about 1 1/2 cups.
Stir in cream and boil sauce until reduced to about 2 cups.
Reserve sauce in pan.
Preheat oven to 450F. and lightly oil a large shallow baking pan.
Rinse trout under cold water and pat dry inside and out.
Fill cavity of each trout with onion compote and season with salt and pepper.
Wrap 4 slices bacon around middle of each trout, overlapping them slightly and leaving head and tail exposed.
In a 12-inch heavy skillet heat oil over moderately high heat until hot but not smoking and brown trout, 2 at a time, until bacon is completely browned, about 5 minutes, pouring off excess fat between batches and transferring trout as browned with a long spatula to oiled pan.
Top each trout with 2 rosemary sprigs and bake in middle of oven 15 to 20 minutes, or until cooked through.
Pour off remaining fat from skillet and deglaze skillet with reserved sauce over moderate heat, scraping up any brown bits.
Strain sauce through a fine sieve into saucepan.
Season sauce with salt and pepper and keep warm.
Divide sauce among 6 plates and top with trout.
How many whole fish can actually fit in a standard 12-inch skillet at once? Two is the safe ceiling. Overcrowding drops pan temperature too quickly, steaming the pork fat instead of crisping it. Trout demand space during the initial sear. Wrapping whole fish in four slices of bacon each creates a dense thermal blanket. That wrapping protects delicate freshwater flesh from direct oven heat while rendering fat directly into the skin. When doubling this recipe for a larger group, do not crowd the baking pan. Keep fish spaced by at least two inches to ensure circulating 450F air reaches every strip of pork uniformly. Cooking times remain stable even when scaling up batches, provided the oven maintains its target heat. Leftover stuffed trout rarely survives well as a direct reheat because the delicate protein dries out instantly in a microwave. Instead, flake chilled leftover meat away from the bones and fold it into cold pasta salads or warm cream sauces on day two. Vinegar-based onion compote scales down cleanly if halved, but always maintain the ratio of liquid to sugar so the reduction thickens properly. Strain reductions through a fine sieve to catch stray rosemary leaves before serving over plates. Deglazing the hot skillet with the reduced broth mixture lifts fond that adds significant depth to the final cream sauce.