There's an art to building the best sandwich, and this recipe is the pièce de resistance. Spreading the salted chive butter on both sides of the freshly cut baguette is about more than just gilding the lily-it protects the bread from getting soggy when layered with the tomatoes. A nutty alpine cheese like Gruyère or Comté balances the acidic tomatoes, while a tangle of baby arugula offers peppery bite. But the most exciting addition to this layered masterpiece might be the fried onions, which add texture, richness, and umami to each bite.
Refrigerate chive butter in an airtight container for up to 5 days. Let come to room temperature before using.