Marinate the chicken breast in the Italian vinaigrette for 1 to 2 hours.
Place chicken breasts on hot grill sprayed with vegetable oil. Turn them after 3 to 4 minutes. Cook until firm but not dried out. Place on platter and keep warm.
In a sauté pan, sauté the onion and garlic until onion is translucent. Do not let the garlic brown or it will become bitter.
Add the Morels and the marjoram, lightly sauté, season with salt and pepper.
Deglaze the pan with the sherry and simmer until the liquid has reduced by half. Whisk in the soft butter a little at a time until well combined. Check the seasoning and keep warm until needed.
To serve, slice the chicken breast and fan out the slices. Top with the mushroom sauce.
A nice accompaniment is grilled asparagus, which you can grill at the same time as the chicken. Marinate the asparagus separately in some of the vinaigrette prior to grilling.
Makes 6 servings.