Heat a large pot of salted water to boiling for the fettuccine.
While the water heats, lightly season the chicken tenderloins on both sides with a bit of salt and pepper and set aside.
Heat 4 tablespoons of the butter in a large pan over medium heat. Add the shallots and cook for a minute or two, just until they are soft and aromatic. Add the chicken to the pan in a single layer and cook, turning several times until light golden brown on the outside and no longer pink in the middle, about 4 to 5 minutes total. Transfer to a plate and cover to keep warm.
Add the raspberry vinegar and water to the pan and simmer for 2 to 3 minutes until some of the liquid evaporates and the sauce starts to thicken. Whisk in the half-and-half and continue cooking until the mixture is smooth and has thickened to a cream sauce consistency, about 3 more minutes. Taste and add salt and pepper as desired.
Remove the sauce from the heat, add the chicken back to the pan along with any juices that may have accumulated. Turn the chicken pieces with tongs to coat them with the sauce. Sprinkle the chives and about 3/4 cup of the raspberries over the top, toss again, then cover to keep warm.
Cook the fettuccine according to the package directions, drain, toss with the remaining 2 tablespoons butter and season to taste with salt and pepper. Divide the fettuccine between four plates and top each with 3 pieces of chicken. Spoon some of the sauce over each plate and garnish with one or two of the remaining raspberries and some whole chives if desired.
Tips for Making This Recipe
If you can't get fresh raspberries, frozen will do in a pinch, but be sure to buy them whole without any syrup and defrost them first.