Preheat oven to 425 degrees F. Spread broccoli and grapes in a single layer on a 15x10x1-inch baking pan. Drizzle with olive oil, salt, and pepper and toss to coat. Roast, 20 to 25 minutes or until broccoli is starting to brown and grapes are soft, stirring once halfway through roasting time. Remove from the oven and sprinkle with red onion. Cool slightly. Meanwhile, in a small bowl whisk together the mayonnaise, lemon juice, and sugar. Place broccoli mixture in a serving bowl. Toss with the dressing and top with bacon and toasted almonds, if desired.