Of course Google brings up a ton of recipes if you search the topic.
I translated a Norwegian one, I'd recommend using fresh ginger instead of
cloves and vanilla, but tastes vary.

Here goes:

Sorry about all the detail, those who make pickles know most from before :-)

1 kg small, firm green tomatoes
2 liters water

Vinegar brine:
2,5 dl 7% vinegar
750 g sugar
8 cloves
1 vanilla stick

Cut away the hard stem part. Rinse tomatoes, prick them with a fork.
Bring water to a boil, add tomatoes and simmer 5-10 minutes until
half tender.

Drain tomatoes and skin them, put on kitchen paper to drain excess water.

Combine vinegar, sugar and cloves in a saucepan. Split the canilla stick
lengthwise and add to pot. Bring to a boil while stirring to dissolve sugar,
add tomatoes and simmer until tender.

Do not cook tomatoes too long, or they will fall apart. Remove tomatoes
carefully, reduce the brine for about 10 minutes, skimming if necessary.
Put tomatoes back in the sauce and bring to boiling point.

Put tomatoes in pickle jars, pour over sauce and close jars.
After one week, carefully pour out the sauce from the jars, boil it again
for 10 minutes, let cool and pour back into jars when cold. Close jars
and store in a cool, dark place.