Step 1

Place racks in upper and lower thirds of oven; preheat to 425°. Cut 1 baguette in half lengthwise, then crosswise. Mix ½ cup (1 stick) unsalted butter, room temperature, 3 garlic cloves, finely grated, and 1 Tbsp. extra-virgin olive oil in a small bowl. Spread over cut sides of bread. Top with ⅓ cup finely grated Parmesan; season with kosher salt and freshly ground pepper. Place bread, cut side up, on a rimmed baking sheet; set aside.

Step 2

Mix 1 large egg, 1 cup panko, 1 cup finely chopped tender herbs (such as basil and/or parsley), 2 tsp. Diamond Crystal or 1 tsp. Morton kosher salt, remaining 1 garlic clove, finely grated, and 3 Tbsp. water in a large bowl; season with lots of black pepper. Add 1 lb. ground beef and gently mix together with your hands. Form into 1½"-diameter balls (you should have 16 total).

Step 3

Cut half of 3 pints cherry tomatoes (about 5 cups) in half. Place all of the tomatoes in a medium bowl; add 1½ cups sliced roasted red peppers from a jar. Drizzle with oil, season with salt and black pepper, and toss to coat.

Step 4

Place 1 red onion, coarsely chopped, on a second rimmed baking sheet; drizzle with oil. Season with salt and black pepper; toss to coat. Arrange in a single layer; roast on top rack until starting to brown, about 5 minutes. Add tomato mixture and roast until tomatoes soften, 10–12 minutes.

Step 5

Place meatballs throughout vegetables; drizzle with oil. Roast, shaking pan once, until meatballs are cooked through and most of the tomatoes have burst, 9–11 minutes.

Step 6

Meanwhile, bake reserved garlic bread on bottom rack until golden brown, 8–10 minutes.

Step 7

Divide meatball mixture among plates and top with basil leaves and Parmesan. Serve with garlic bread.