Cut rosemary sourdough bread in half and hollow out loaf, leaving 1/4 to 1/2 inch of bread around the edge. It's best to use a paring knife to cut along the inside and then use your hands to pull out just enough of the inside of the loaf.
Combine garlic, peppers, mushrooms, olives and celery in a sauté pan, and sauté until soft.
Combine burger mix and hot water, mix and let sit for 5 minutes (as directed on package). Then combine contents of sauté pan with mix. This should be a fairly thick consistency, if not, let sit a few minutes to let water absorb. Stuff the hollowed out loaf with the mix. Wrap stuffed loaf in tinfoil and bake at 350 F/ 175 C for 30 minutes.
Allow to cool, and refrigerate (this is where it really sets up). Reheat in the oven on a low setting for 20 minutes wrapped in tinfoil, then uncover for last 5 minutes to get the crust crispy.
Slice and serve! (Cranberry sauce or mushroom gravy also pair with this very well)