Toss the broccoli florets and red onion with the olive oil and a couple pinches of salt and spread out on a sheet pan. Roast until browned and crisp, 18 to 22 minutes.
Meanwhile, combine the raisins and sherry vinegar in a small saucepan and bring to a simmer. Remove from the heat and allow the raisins to plump for 10 to 15 minutes.
For the dressing: Combine the mayo, milk, sugar, vinegar, a pinch of salt and a bunch of black pepper in a large bowl.
For the salad: Add the broccoli mixture to the dressing along with the grapes and celery. Drain the raisins, then add those. Fold everything together until combined, then taste and adjust seasoning. If not serving right away, cover and refrigerate. Top with the almonds right before serving.