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 to a medium saucepan and press as much of the fennel below the oil as possible (the oil will barely cover everything at first but it will cook down later). Bring to a simmer over medium heat, cover, and reduce to low to maintain a very low simmer. Cook for 1 hour, stirring occasionally, or until the flavor the oil has reached your liking. Remove from heat and strain through a fine mesh strainer set over a medium mixing bowl. Allow to cool to room temperature, transfer to an air-tight container and store in the refrigerator.
I tried a batch using only a few orange zest strips and didn't get a lot of orange flavor. The next batch I increased it to 6 strips and the orange flavor was pronounced. Hopefully that helps you decide how many to use.
I tried a batch using 1/2 teaspoon red pepper flakes and didn't get much of any heat. The next batch I increased it to 1 teaspoon and while not spicy, there is a little kick on the finish.
I haven't tried cooking the oil any longer than 1 hour as I assumed all of the fennel flavor has been extracted at that point. But I would love to know the results if you cook it longer!