Mother's Day dinner

You could even do a white gazpacho--I love how elegant and surprising it is (not that regular gazpacho isn't elegant...).
A simple plate of butter-braised veggies is never amiss--asparagus, small new or fingerling potatoes, green beans, carrots, etc.
Radishes with butter is another go-to appetizer of mine. Especially if there's rustic bread around. Or, with that rustic bread, have some tapenade.
Another possibility for a side dish could be a grain-based salad--quinoa, wheat berries, or spelt with lightly steamed veg and vinaigrette.

It's a fish, popular in Italy (as well as other European countries.)

I agree gazpacho would be a good start - I love cold soups in the spring/summer. Another idea: I recently made Nancy Silverton's Caponata from her Mozza cookbook - best one I've found yet. Best served at room temp/cool, with crusty bread. Really delicious and I think it would work well before the branzino: http://www.wherela.com/chefspicnic/recipes/silverton/silverton.pdf

What is branzino?

How about a gazpacho soup to start. There are a number of great recipes here:
http://food52.com/recipes/search?q=gazpacho