Cook Mushrooms
Heat 2 teaspoons olive oil in a large skillet over medium-high heat. Add mushrooms to oil and cook, stirring occasionally, until browned on both sides. Season mushrooms to taste with salt and pepper. Set aside to cool slightly. 

Make Olive Salad
While the mushrooms are cooking, start the olive salad:

1. Coarsely chop giardiniera and place in a medium bowl. 

2. Smash olives with the side of a chef's knife, remove pits, and roughly chop. 

3. Add to giardiniera. 

4. Add capers, garlic, spices, olive oil, and red wine vinegar to giardiniera.
   
   Mix well, then set aside.

Assemble Sandwich

1. Cut bread in half. If your loaf is very thick, remove some bread from inside the loaf, leaving about a 1-inch shell. 

2. Brush bottom of the bread with some of the dressing from the olive salad, then cover with half of the olive salad. 

3. Cover with all the roasted peppers, then top with all the artichoke hearts. 

4. Cover portabellas with remaining olive salad. 

5. Brush bread top with any leftover dressing, then place on sandwich, gently pressing down on loaf.

6. Cut muffuletta into wedges and serve immediately or wrap tightly in plastic wrap and refrigerate until ready to eat.