I make vegetarian muffalettas all the time. A couple suggestions: (1) Make the "olive salad" about a week in advance so it will be more flavorful. Also, my olive salad uses olive brine (juice) rather than the giardiniera brine, though I chop up drained giaridiniera and use in the olive salad . I also add some caper brine. I also add more things to my olive salad: olive oil, lots of thyme (dried, rubbed in your hand), oregano (dried, rubbed in your hand), chopped fresh celery, garlic clove(s) in slivers. I suppose you can make the flavored oil if you like, as included in this recipe. (2) Slice and saute your mushrooms -- lots of them. Much more umami. I use a mix, including white button mushrooms, portobello mushrooms, and any others you like. (3) In assembling the sandwich, use a solid crusty loaf, cut so it has a deep bottom layer, dig out the bottom fleshy part of the bread, and put the olive salad and sauteed mushrooms there (it won't fall out all over the place as badly). If afraid that the bottom will be soggy, start with provolone cheese on the bottom then olive salad and top with another slice of provolone on top before baking. Delicious!