Welshstar, if I were you I would save the raspberries for a dessert like pannacotta or icecream with a raspberry coulis. Quite easy to make and delicious.
Seared tuna asks for something simple but tasty. How about a "sauce vièrge", or -in english- a virgin sauce. Perfect match with seared tuna. BTW, before searing tuna, put a good handfull of white sesame seeds on a plate and dip both sides of the tuna in untill nicely covered. Sear tuna in a very hot pan. Looks nice, tastes great!!
A sauce vièrge is so basic, I use it frequently with tuna, and, I haven't met anyone who didn't like it. The base is diced tomatoes, diced shallots, basilleaves, lemonjuice and olive oil. You could add white balsamico instead of lemonjuice, chopped parcely and/or cilantro, and finally olive oil. It is served luke warm.
For instance; Peel 3 tomatoes; make a cross incision in the skins and put the tomatoes 10-15 seconds in boiling water. Take skins off. Cut the tomatoes in 4 and remove all the seeds. Now cut the remaining tomatoflesh in small dices.
Chop 2 shallots finely. Chop a handfull leaves of basil and/or cilantro and/or parcely. Add juice of 1 lemon or 1 tbsp of white balsamico (I'd go for lemon with a few drops of balsamico). Don't use regular balsamico, it will look ugly!
Put everything in a heatresistant non-metal bowl with some S&P. Mix gently. Now heat about a cup of olive oil untill very hot. Carefully pour the very hot oil in one go on the mixture. Done! The longer it sits, the better it gets, so you can make it a while before and if you like just warm it a little bit to serve.
The nicer you cut the ingredients, the nicer the sauce will look.
PS Drink the champagne instead of mixing it in a sauce. Enjoy your evening!