Bring tomatoes, sugar, vinegar, coriander, salt and pepper flakes to boil in Dutch oven over medium-high heat. Reduce heat to medium-low and simmer, stirring occasionally, until mixture thickens to jamlike consistency and rubber spatula or wooden spoon leaves distinct trail when dragged across bottom of pot, about 45 minutes to an hour. Stir in lemon juice.
Transfer chutney to a pint jar with a tight-fitting lid and let cool to room temperature before refrigerating. Chutney can be refrigerated for up to 1 month.
A couple substitutions you can make:
If you wish to use normal green tomatoes, be sure to cut them up into 1-inch pieces before cooking
The original recipe calls for using white distilled vinegar, so feel free to use that or another favorite vinegar, in place of the apple cider vinegar
A note on the spicing - this recipe only includes coriander seeds and a pinch of red pepper flakes. I liked the simplicity both in terms of preparation and in terms of how the flavor balanced with the sugar and vinegar. However, my husband found it a bit one-sided. So if you want to boost the flavor profile, add other common chutney spices, like mustard seeds, ginger, or cinnamon.
Add all ingredients except the oil to a blender or food processor and process until pureed. With the motor running, add the oil in a thin stream and process until emulsified. It will keep for 1 week in the refrigerator in an airtight container.