Look for celery root that is firm—not spongy—at the bottom end.
- 2 slices thick country-style bread, torn into bite-size pieces
- 1 anchovy fillet, chopped
- 1 small garlic clove, chopped
- 1 tablespoon fresh lemon juice
- 1½ teaspoons white wine vinegar
- Freshly ground black pepper
- 1 ounce Parmesan, grated (about ⅓ cup), plus shaved for serving
- 2 romaine hearts, leaves separated
- 8 celery heart stalks, thinly sliced, plus all leaves
- ¼ large celery root (celeriac), peeled, cut into matchsticks
Preheat oven to 400°. Toss bread with olive oil on a rimmed baking sheet, squeezing to help it absorb the oil; season with salt. Toast, tossing halfway through, until golden and crisp, 5–10 minutes. Let croutons cool.
Blend egg yolk, anchovy, garlic, lemon juice, vinegar, and mustard in a blender until smooth. With motor running, gradually drizzle in vegetable oil, blending until thickened. Dressing should be pourable (add a splash or two of water to thin, if needed).
Transfer dressing to a bowl large enough to hold salad and season with salt and pepper. Mix in grated Parmesan. Add romaine, celery stalks and leaves, and celery root; toss to coat.
Serve salad topped with croutons and shaved Parmesan.
DO AHEAD: Dressing can be made 2 days ahead. Cover and chill. Bring to room temperature before using.