Heat oven to 400° with rack in middle. 

 

Fill a medium bowl with ice and cold water. Submerge onion in the ice water; let sit for 20 to 30 minutes. 

 

Meanwhile, in a large bowl, whisk together 2 tablespoons oil, lemon juice and generous pinch salt and pepper. Add bread and gently toss to coat. Spread bread in 1 layer on a baking sheet and toast, stirring once, until golden, about 10 minutes. 

Remove croutons from oven and cool in pan on a rack. 

 

Peel celery, then cut stalks crosswise into three pieces each. Very thinly slice pieces lengthwise. Using vegetable peeler, cut cucumber pieces into long strips to form ribbons. 

 

In a bowl, whisk together vinegar, mustard and pinch salt. Whisking, add remaining 1/4 cup oil in a slow, steady stream. 

 

Drain onion and pat dry with paper towels.

 

Divide lettuces, onion, celery, cucumber, croutons and tongue among 4 serving plates. Drizzle with dressing, and sprinkle with salt and pepper.