In large bowl, combine onions, garlic, oil and rosemary; salt and pepper to taste.
Transfer to a large baking sheet. Bake in a preheated 450°F oven 25 minutes or until golden brown, stirring halfway through.
Meanwhile, in a large saucepan, heat butter over medium heat. Whisk in flour; cook 2 minutes, whisking constantly.
Whisk in half & half; bring to a boil; reduce heat and simmer 1 minute. Add roasted onions and garlic. Toss to coat; salt and pepper to taste.