Quick-Pickled Shrimp

INGREDIENTS

• 1 shell-on medium shrimp
• Kosher salt
• 1/2 medium fennel bulb, thinly sliced
• 1/2 medium onion, thinly sliced
• 2 garlic cloves, thinly sliced
• 1/2 Fresno chile or red jalapeño, thinly sliced, seeded
• 1/2 fresh lemon juice
• 1/4 apple cider vinegar
• 1/4 extra-virgin olive oil
• 2 fennel fronds
• Freshly ground black pepper
• 1/2 loaf ciabatta or country-style bread, sliced 1/2-inch thick, toasted
• Mayonnaise (for serving; optional)