Make the celery salt: Preheat the oven to 300º. On a baking sheet, spread the grated celery root in an even layer. Transfer to the oven and bake until the celery root is completely dry and slightly golden, 30 minutes. Let cool completely, then transfer to a spice grinder with the flaky sea salt. Grind into a smooth powder.
Make the bread crumbs: Increase the oven temperature to 350º. In a small saucepan, combine the olive oil and the garlic cloves over medium heat. Cook until the garlic is golden and tender, 8 to 10 minutes. Using a slotted spoon, transfer the garlic to a food processor with ¼ cup of the garlic oil, the rye bread and salt. Pulse into coarse crumbs, then transfer to a baking sheet. Bake until golden brown, 10 to 12 minutes, then let cool completely.
Make the sorrel dip: In a blender, combine the sorrel leaves, yogurt and goat cheese, then blend until smooth. Transfer to a medium bowl and fold in the fromage blanc and lemon juice. Season with salt, then transfer to a serving bowl and garnish with the bread crumbs.
Serve the dip alongside the breakfast radishes garnished with celery salt.