• Adjust rack to top position (top and middle positions for 4 servings) and preheat oven to 425 degrees. Wash and dry all produce. • Halve, peel, and thinly slice half the onion (whole onion for 4). Cut potatoes into 1/2-inch-thick wedges. Halve bell pepper lengthwise; remove stem and seeds. • Toss sliced onion on a piece of foil with a drizzle of olive oil, half the vinegar (all for 4), salt, and pepper; cinch into a packet.

• Toss potatoes on one side of a baking sheet with a drizzle of olive oil, half the garlic powder (you’ll use more later), salt, and pepper. • Place bell pepper and onion foil packet on empty side. Drizzle bell pepper with olive oil and season with salt and pepper; rub to coat. Arrange cut sides down. (For 4 servings, toss potatoes on one sheet; place bell peppers and onion foil packet on a second sheet.) • Roast on top rack until bell pepper and onion are browned and tender, 15-20 minutes (you’ll roast potatoes a bit longer). (For 4, roast potatoes on top rack and bell peppers and onion on middle rack.)

• Once bell pepper and onion are done, transfer to a cutting board. • Give potatoes a toss. Return to top rack until browned and crispy, 5-10 minutes more. • Turn off oven; leave potatoes inside to keep warm until ready to serve.

• Meanwhile, pick parsley leaves from stems; mince leaves. Thinly slice chives. • In a small bowl, combine mayonnaise, minced parsley, chives, and a big pinch of remaining garlic powder to taste. Season with salt and pepper.

• Finely chop bell pepper and balsamic onion together to form a relish. Season with salt and pepper. • Spread half the sourdough slices with relish; sprinkle with mozzarella. Spread remaining slices with half the herb aioli. Top mozzarella with remaining slices, aioli sides down.

• Heat a large drizzle of olive oil in a large pan over medium heat. Add sandwiches and cook until bread is golden brown and cheese has melted, 4-6 minutes per side. (For 4 servings, you’ll need to work in batches or use a second pan.) TIP: If necessary, add another drizzle of olive oil after flipping melts. • Halve melts on a diagonal. Divide between plates and serve with potatoes and remaining herb aioli on the side.