This is a good use for end-of-season green tomatoes, and this chutney just might go well on a hamburger.
Place a large stockpot of water over high heat, and bring to a boil. Cut a 1-foot square of cheesecloth. Place mint in center, and form a bundle; secure with kitchen twine, and set aside. Prepare an ice bath, and set aside.
Remove core at stem end of each tomato, and cut an "X" in the skin at the other end. Drop tomatoes in boiling water, 3 at a time, and cook until skins begin to loosen or crack, about 8 minutes. Plunge into ice bath. Using a paring knife, peel off skin and discard. Repeat with remaining tomatoes. Cut tomatoes into 3/4-inch chunks, and set aside.
Combine onions, vinegar, mint bundle, sugar, salt, raisins, and 1 cup water in a low-sided, 6-quart saucepan. Set over medium-high heat, and bring to a boil.
Add tomatoes, and reduce to a simmer. Cook, stirring frequently, until the tomatoes are tender, about 1 hour.
Increase heat to high, and continue cooking, stirring frequently, until almost all liquid has been absorbed, about 5 minutes. Remove pan from heat, and discard the mint bundle.
Transfer to a large bowl set over an ice bath to chill; store in airtight container, refrigerated, up to 4 weeks.
For the most mint flavor, press on the mint bundle with the back of a spoon while stirring.