My favorite way of preserving tomatoes is making ketchup :)
I have this simple recipe that I want to share with you:
about 2 kilos tomatoes
8 medium size onions
1 tablespoon salt
5 tablespoons sugar
1/2 teaspoon pepper
2 teaspoons mustard
150 ml vinegar
Cut tomatoes and onions into pieces and cook them with all the other ingredients in a big pot.
When everything is soft-boiled, use a blender shaft to mash it up.
Cook it further for about 15 minutes, if you want to have a more compact mass, cook it longer.
Then fill it in sterile jars (I use screw top jars), close them and put them upside down until they are cooled off.
It's really delicious in combination with roasted vegetables!