Cover the potatoes and celery root with cold water in a large pot and bring to a boil. Boil 20-25 minutes, until soft. Drain.
Return potatoes and celery root to pot. Add milk and butter and mash until smooth. Season with sea salt and freshly-ground black pepper. Transfer mixture to a bowl and let cool.
Heat 2 Tbsp. Olive oil in a large saute pan over medium heat. Add onion and garlic and saute until soft. Add spinach and saute until wilted. Season with sea salt and freshlyground black pepper. Add to potato and celery root mixture and mix well.
Add egg, flour and cheese to potato, celery root and spinach mixture and mix well.
Heat 2 Tbsp. Olive oil in a large saute pan. Take 1/4 cupfuls of the mixture and place in pan. Cook until brown on each side, 3-4 minutes per side.
Cook rest of patties in remaining olive oil.
Serve with aioli, recipe below.
FOR THE AIOLI
2 egg yolks, at room temperature
3 cloves garlic, peeled and grated
1 1/2 Tbsp. White wine vinegar
2 cups Extra Virgin olive oil
1 tsp. Salt
Freshly-ground black pepper
1/4 tsp. Smoked paprika
Pinch of cayenne pepper
Combine egg yolks, garlic, vinegar in a large bowl. Whisk until mixture is foamy.
Gradually add olive oil, whisking continuously until mixture thickens to the consistency of mayonnaise.
Season with salt, freshly-ground black pepper, smoked paprika and cayenne pepper.