This deliciously simple compound butter oozes with summery zest. Let a pat melt over grilled fish or slather it on boiled sweet corn. Use very ripe, very red tomatoes for best color and flavor.
Place the tomatoes in the bowl of a food processor and process until puréed, about 1 minute. Add the butter, salt, and pepper. Process until blended, stopping occasionally to scrape down the sides of the bowl; this may take 2 or 3 minutes.
Stir in the tarragon. Scrape into a small bowl and cover. Or, make a log of butter by spooning it onto a piece of waxed paper, folding paper over the butter and rolling it gently until you have a sausage shape, then twist the ends to seal. Refrigerate for at least 2 hours and up to 4 days. You can also wrap the log in foil and freeze for up to 3 months; thaw it in the refrigerator.