Preheat oven to 350 degrees. Oil two non-stick muffin pans with oil or oil spray. Set aside.
Now it's time to make a flax egg. In a small bowl, whisk together water with flax meal. Put it into the refrigerator for at least 10 minutes to set up. It will become thick and gooey.
Bring a non-stick skillet to a medium heat with a teaspoon of oil. Saute celery, carrots, onions, and garlic for a few minutes, until translucent and fragrant.
Add chopped chestnuts to the skillet and cook for a couple minutes more.
In a large mixing bowl combine dried bread cubes with mixture from skillet.
In a large measuring cup, combine vegetable broth with dried rosemary, thyme, sage, ½ teaspoon salt, and a few grinds of black pepper. Use a fork or whisk to blend the spices with the broth.
Remove the flax egg from the refrigerator. Put half of the mixture on top of the bread cubes.
Slowly pour the broth into the mixing bowl with bread cubes and vegetables. Occasionally stir the bread cubes, so that the cubes get evenly coated. Halfway through, add in the second half of the flax egg and combine. You want the cubes to be moistened, but not wet or mushy. There shouldn't be any liquid at the bottom of the bowl. So if it looks like the bread cubes are getting overly wet, stop there. However, if the cubes look too dry after all of the liquid has been poured, add a few more splashes of water or broth. Because you want the cubes to stick together in a muffin shape, you'll want the cubes slightly more damp than what you'd have for regular stuffing.
Taste for salt, and add up to ½ teaspoon more if necessary.
Now it's time to make the stuffing muffins. Use your hands to make a small ball of stuffing, and press it into one of the muffin cups in the tin. Continue until all of the stuffing is done. (If you'd rather not use your hands, you can use an ice cream scoop or spoon. Just make sure that the mixture gets fully pressed together in the tins.) You will likely not fill both muffin pans all the way. It makes around 20 muffins.
Put the muffin tins into the center of the oven. Bake for 25 to 30 minutes, until the tops are nutty brown. Remove the muffins from the oven. Allow them to sit in the muffin tins and cool for 10 minutes. Otherwise, they won't be easy to remove and will be more likely to fall apart.
Once the muffins have cooled, use a butter knife to carefully go around the edges of each stuffing muffin to unstick them from the sides of the tin. Then carefully remove each one. (If some of the muffins come apart a bit as you remove them, don't stress. They will still be delicious!)