Place the shredded 2 cups red cabbage in a bowl and sprinkle with a pinch of salt.
Let it sit for 10–15 minutes, then gently massage and lightly squeeze.
While the cabbage rests, peel and julienne the celery root, julienne the apple (no need to peel), chop the dill.
Place the ¼ cup sunflower seeds in a dry skillet over medium heat.Toast, stirring frequently, for 3–5 minutes, until they turn lightly golden and smell nutty and fragrant.
In a small bowl, whisk together 3 tablespoons full-fat Greek yogurt, 1 tablespoon sour cream, 1½ tablespoons fresh lemon juice and 1 teaspoon lemon zest, 1 teaspoon honey, 1 teaspoon Dijon mustard, ¼ teaspoon horseradish, and ¼ teaspoon celery seeds. Slowly whisk in 1 teaspoon neutral oil until the dressing is smooth and lightly creamy.
Season with salt and white pepper. Add the 1½ cups celery root and 1 Granny Smith apple to the softened cabbage.
Pour the dressing over the mixture and toss until everything is evenly coated.
Fold in most of the 1.4 tabelspoons dill and sunflower seeds, saving a little for the top.
Taste once more and adjust the seasoning if needed. Top with the remaining 0.6 tabelspoons dill and seeds before serving.
Notes
Lemon is your best friend, use it early - the moment celery root is peeled and cut, toss immediately with lemon juice.Salt the cabbage first, then squeeze - toss shredded red cabbage with a pinch of salt and let it sit 10 minutes. This softens the raw edge without losing the crunch.Celery seeds — crush them - thy will release their aroma. Whole seeds can taste bitter.