Regarding this subject, I do things differently than the popular consensus of the internet tells me to do and I've been very happy with the results. Here is how I do a Flanders with fruit:
Brew a simple batch sparged Belgian amber ale mashed at 152F. Zero hops. 4oz. of malto-dextrin added to the boil. Chill to the low 60s. Pitch one pack of Roeselare WY3763. No starter. Pitch one pack of brett or a small starter of dregs from a brett beer.
Let it ferment for 30 days. It should get bubbling in less than 2 days. After 30 days, check the gravity. It should be around 1.010 already. Now add the fruit to the primary. That's right, I said add the fruit after 30 days. Let it ferment for another 5-6 months. Don't open the fermenter during this phase. There is no reason to. Satisfying your curiosity too see how sour it is won't make the beer taste better when you are ready to drink it, but it might make it taste worse.
After 6-7 months of fermentation and 5-6 months on the fruit, you should be ready to bottle. Check the gravity. It should be around 1.002-1.006. I've had good luck not adding wine yeast at bottling. However, it takes 1-2 months for the bottles to carb. During this time, the sourness increases.
9 months after brewing, you should have a tasty fruited Flanders fully carbed in bottles. The Flanders I currently have in bottles used this method with 1.5# of dried black currants. The fruit flavor is substantial. The internet says it shouldn't be. The sourness is mouth watering. The pH is 3.32. The brett character is robust.
Awesome! Thanks for sharing! I'm just starting to get my sours up and running, so this is good info for me.
Here's a few followup questions, if you don't mind. It sounds like you're not adding any priming sugar - do you find that the 1.002-1.006 range is the right target for bottle-conditioned sours? Do you ever have any issues with overcarbonated bottles at the upper end of that range, or undercarbonated at the lower end? Are you using heavy Belgian bottles, or the normal 12oz and bombers that we typically recycle from US craft brewers?
For the black currants, are you using 1.5lb in a full 5-gallon batch? I'm planning on using my harvest this season in a sour or two (a sour-wort Berliner Weisse and/or part of a batch of Lambic I brewed this fall). I was thinking of using 2-3 pounds in a 3-gallon batch. It sounds like you are using a whole lot less.