Drain the kidneys well and pat dry if they were soaked. Preheat the oven to 325F. In a heavy baking pan like a dutch oven, warm a few tablespoons of the lard or cooking oil. With the bacon and cook on medium heat to render out it’s fat. Remove the bacon to a bowl. Season the diced meat and kidneys with salt and pepper liberally.
Toss the kidneys and meat in flour, keeping them separate. First brown the meat, working in small batches on medium heat, and transferring browned meat to the bowl with the bacon. If the pan or oil starts to look too dark, clean it by adding a splash of water off of the heat, scraping the brown bits up and adding the meat that’s already seared, then proceed with browning until all of the meat and then kidneys are deeply browned.
When the meat is browned, add a little more lard or oil to the pan if needed and sweat the onion in the drippings for a few minutes until tender. Add the thyme to the onions. Deglaze the pan with the stout, cook for 3-4 minutes more, then add the browned meat-kidney-bacon mixture, along with any juices, and stock. Cover the pot and bake for 1 hour.
Remove the pot from the oven and carefully transfer the meat and juices to a container, then cool to room temperature, taste the seasoning and adjust as needed, then refrigerate overnight to let the flavors meld. The juices will set up and should be completely firm when cold.