In a large skillet melt 2 tablespoons of the butter and cook the mushrooms, seasoned with salt and pepper, over moderate heat, stirring, until the liquid evaporates. Transfer the mushrooms to a bowl and allow to cool. Melt another 2 tablespoons of butter in the same skillet and cook the spinach with salt and pepper, over moderately high heat, stirring, until wilted. Transfer the spinach to a small colander to drain and cool.
Arrange the fillets, skinned sides up on a work surface and season them with salt and pepper. Cover each fillet with 1/4 of the cooked spinach, arrange a half roasted red pepper on each fillet and divide the mushrooms among the fillets. Beginning with the narrow (tail) end of each fillet, roll up the fillets jelly-roll fashion to enclose the filling. Cut each rolled fillet crosswise into thirds and secure each slice with a wooden toothpick.
Arrange the roulades cut sides up in a buttered deep skillet just large enough to hold them in one layer. Add the stock, wine and water to the skillet and poach the roulades, covered, at a bare simmer for 5 to 6 minutes, or until they just begin to flake when tested with a fork. Transfer the roulades to a paper towel-lined plate to drain.
In a bowl whisk together the vinegars, mustard, the shallot, and salt and pepper. Gradually whisk in the oils, in a stream, until dressing emulsifies. Add the fresh herbs. Transfer the vinaigrette to a saucepan and gently heat, swirling the pan, until just warm.
Arrange the roulades on 4 dinner plates and spoon the warmed vinaigrette around the roulades.