Puree cucumber and tomatoes with 1/2 cup water, and any dried herbs if desired. (I used a dash each of marjoram, parsley, and savory, though next time I would leave the savory out and add thyme and rosemary instead. I would have used dill but we didn't have any.)
Add flour, yeast, and salt, to the bowl of a stand mixer fitted with the dough hook attachment, and turn on briefly to mix.
Pour the cucumber mixture into the bowl, and run the mixer on Speed 2 for at least five minutes or until well incorporated, with no dry spots of flour left. You may have to pause to scrape the bowl or stir it manually to help things along. Add more water, 1/4 cup at a time, if it looks dry. The mixture should be quite wet, kind of like a no-knead dough.
Cover and leave to rise for 2–4 hours. I think I rested it for 3 hours? Not too sure of the exact time, but basically let it sit until it's grown to roughly 1.5× in size. It ferments quite slowly due to the acidity of the tomatoes.
Scoop or pour the mixture into two small loaf pans, or one large one. It won't rise too much in the oven, so feel free to pour it 3/4 full. Transfer to a cold oven and let rise again for about 30 minutes, or until it's reached close to the top of the pan.
With loaves inside the oven, preheat to 400 °F and set a timer for 40 minutes. After 40 minutes, check every 10 minutes until crust is browned to your liking.
Cool cucumber bread completely before slicing.