1/2 Fresno chile or red jalapeño, thinly sliced, seeded
1/2 cup fresh lemon juice
1/4 cup apple cider vinegar
1/4 cup extra-virgin olive oil
2 tablespoons fennel fronds
Freshly ground black pepper
1/2 loaf ciabatta or country-style bread, sliced 1/2″ thick, toasted
Mayonnaise (for serving; optional)
Cook shrimp in a large pot of boiling salted water until just opaque in the center, about 2 minutes. Drain; rinse under cold running water to cool. Peel and devein, leaving tails intact, if desired.
Combine shrimp, fennel bulb, onion, garlic, chile, lemon juice, vinegar, oil, and fennel fronds in a medium bowl. Season with salt and pepper and toss to combine. Let sit, tossing occasionally, at least 20 minutes.