Toasting walnuts while browning the butter gives this warm vinaigrette an intense nuttiness and luxurious richness. Garlic, rosemary and lemon zest add layers of bright, herbal flavor and fragrance. This is perfect spooned onto roasted vegetables such as asparagus, green beans, carrots, broccoli, cauliflower or Brussels sprouts. Or drizzle it onto seared scallops or delicate fish such as cod or flounder.