• Mix together 2 Tablespoons olive oil, the smashed garlic clove, ¼ teaspoon dried oregano, and ⅛ teaspoon red pepper flakes in a small bowl. Microwave on high for 30 seconds, then stir, and microwave for another 30 seconds. By now, the garlic should just be starting to brown. If not, microwave for one more 30-second interval. Remove the bowl from the microwave and set it aside for now.
• Prep the red bell pepper by cutting off the top and bottom. Cut the center portion of the pepper in half lengthwise and remove the seeds and pith (the white part inside). Carefully remove the stem from the top section. Brush all pieces lightly with olive oil and place them skin-side-up on a foil-lined baking sheet. Place the baking sheet in the top of the oven, about 4 to 6 inches (10 to 15 cm) from the top element, and broil until the peppers are soft and the skin starts becoming blackened in spots, about 10 minutes. Immediately transfer the hot pieces of bell pepper to an airtight container or a plastic-covered bowl. Let it sit while you prep the other vegetables. Turn the oven on to 450°F.
  
• Prep the zucchini by trimming off both ends. Cut each zucchini lengthwise into 3 long planks.
• Prep the portabella mushroom caps by pulling off and discarding the stems. Use a small spoon to scrape off most of the dark brown gills on the underside of the mushroom. Cut the mushroom cap into approximately ½-inch (1.25cm) thick slices.
• Lay the zucchini planks and portabella slices on the same baking sheet you used to broil the bell peppers. Remove and discard the crushed garlic from the olive oil mixture, and drizzle the oil over the zucchini and mushrooms. Sprinkle the vegetables with ¼ teaspoon kosher salt. Transfer the baking sheet to the oven and roast the vegetables for about 10-15 minutes, until the zucchini planks are browned on both sides, flipping the vegetables once halfway through the cooking time.
  
• Meanwhile, after the red bell peppers have been sitting for at least 10 minutes, uncover them and check to see if they are cool enough to handle. If they are, start peeling the skin off each piece. Note that it should come off pretty easily, but if it doesn't, don't worry if you can't get it all off. Cut the pepper into approximately ½-inch (1cm) slices.
• Cut the halloumi into roughly ¼ to ½-inch (about 1cm) thick slices. Heat 1 Tablespoons olive oil in a large non-stick skillet over medium heat. Place the cheese into the skillet and cook until golden brown on the bottom, about 5 minutes. Flip the cheese over and cook until browned on the second side, about 3 more minutes. Remove the skillet from the heat.
• Cut the focaccia in half horizontally. If desired, lightly toast the bread under the broiler (cut sides down so that the insides stay soft). Make sure to keep an eye on the bread so that it doesn't burn or get too toasty. Spread the cut sides with ¼ cup vegetable spread (ajvar) or pesto, dividing it evenly between the pieces of bread.
• Layer the vegetables on the bottom slice of bread, in whatever order you like, making sure that all of the vegetables are evenly distributed. Layer the halloumi slices on top of the vegetables and top with a layer of leafy greens, if desired. Place the top piece of bread on the top of the sandwich. Use a large bread knife to cut the sandwich into 6 rectangles. Serve immediately or wrap individual sandwiches in foil for bagged lunches or a picnic.

Make-Ahead Instructions

• Complete the vegetable prep steps through step 6 of the recipe. Once the vegetables are all prepped, put them into an airtight container in the fridge until you're ready to make your sandwich. Reheat the vegetables for one minute in the microwave before assembling the sandwich. Note that the halloumi should not be seared ahead of time - it's best to sear it right before assembling the sandwich.