Red Pepper–Anchovy Toasts

Name: Red Pepper–Anchovy Toasts
Ingredients: ½ small white onion, halved, thinly sliced; ½ small red bell pepper, quartered, ribs and seeds removed, flesh sliced into thin strips; 3–4 guindilla peppers or other small pickled chiles (depending on heat preference), finely chopped; 1 Tbsp. sherry vinegar or red wine vinegar; ½ tsp. sugar; 4 Tbsp. (or more) extra-virgin olive oil, divided; Kosher salt; ½ baguette, sliced on a diagonal ⅓ thick; 1–2 garlic cloves; 8–10 boquerones (marinated white anchovies), oil-packed anchovies, or other cured fish (from at least a 1.7-oz. container)