Give your roast beef sandwiches a lift with this flavorful roasted onion and olive medley. A heady horseradish-mustard enhanced mayo adds a nice punch, too.

• Position racks in the top and bottom thirds of the oven and heat the oven to 425°F.
• On a large rimmed baking sheet, mix the onion, olives, 1/4 cup olive oil, the rosemary, sugar, and 1/2 teaspoon each salt and pepper. Spread in the middle of the baking sheet. Dredge both sides of the tomato slices in the rosemary-olive oil, and arrange on the sides of the sheet. Roast on the bottom rack for 10 minutes. Remove the tomatoes and stir the onions. Continue to roast the onions until softened and golden, about 20 minutes.
• Put the halved kaiser rolls on another baking sheet, cut sides up, and bake in the upper third of the oven for 10 minutes, adding 4 piles of 3 slices roast beef each to the roasting pan to warm through for the last 5 minutes.
• Meanwhile, put the mustard in a small bowl and slowly whisk in the remaining 1/4 cup olive oil to emulsify. Stir in the horseradish.
• Spread the mustard mixture on the cut sides of the rolls and mound the roast beef on the bottom halves. Season with salt and pepper. Divide the cheese, tomatoes, and onion mixture among the sandwiches and top with the top halves of the rolls. Serve warm.