Our version of a French dip, which is an open-face roast beef sandwich, uses leftover grilled lamb. The meat is served warm on thick country bread.
Preheat oven to 350 degrees. In a large skillet, heat oil over medium heat. Add onions, garlic, fennel seeds (if using), and 2 tablespoons water; season generously with salt and pepper. Cover, and cook, stirring occasionally, until onions are caramelized, 25 to 30 minutes.
Raise heat to high; add vinegar and 2/3 cup water. Boil, scraping up browned bits from bottom of skillet, until liquid is slightly reduced, 2 to 3 minutes. Drain onions in a sieve set over a bowl, reserving liquid.
Arrange bread slices on a baking sheet. Top each slice with lamb and onions, and drizzle with oil. Bake until heated through, about 5 minutes. Serve immediately, with reserved liquid spooned over top.
If the bottom of the skillet begins to blacken while the onions are cooking in step 1, add a splash of water, then scrape the bottom of the pan with a wooden spoon.