Pick the red onion. Peel it and slice into thin half-moons, about ⅛-inch thick. Place in a heatproof bowl with vinegar, sugar, and salt. Stir until dissolved and set aside for at least 5 minutes (or longer). They'll soften slightly but still hold their shape.
Prepare the sardines. Pat them dry thoroughly—this helps the skin crisp up. Season generously on both sides with salt and pepper. If your fishmonger has left the scales, give them a quick scrape; otherwise you can eat them (they're fine). Remove any large bloodline running along the spine if visible.
Toast the bread. Heat 1 tablespoon olive oil in a cast-iron or heavy skillet over medium-high heat. Rub one side of each toast with the cut garlic clove, then place them skin-side down in the hot pan. Cook for about 2 minutes until golden and crisp. Flip and cook another minute. Remove to a plate.
Cook the sardines. Add 1 tablespoon olive oil to the same skillet (no need to wipe it out). When the oil shimmers, lay the sardines in—skin-side down if they have skin, otherwise just one side at a time. Don't crowd them; cook two at a time if needed. Let them cook undisturbed for 2 minutes so the bottom crisps slightly. Flip and cook another 1-2 minutes until the flesh is opaque throughout (the internal temperature should reach about 145°F / 63°C).
Serve. Place each sardine on top of a piece of toast, skin-side up if you prefer it visible. Drizzle with any remaining olive oil from the pan and scatter the pickled red onion over the top. Serve immediately while the fish is still warm.
