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1
Heat the oven to 250 degrees F and arrange a rack in the middle.
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2
Set a wire rack over a baking sheet and set aside.
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3
Using the coarse holes on a box grater or the medium-coarse shredding disk of a food processor, shred the celery root and onion and place in a large bowl.
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4
Add the eggs, matzo meal, and measured salt and pepper and stir to combine; set aside.
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5
Heat the oil in a large frying pan over medium-high heat until a piece of celery root instantly sizzles when dropped in, about 4 to 5 minutes.
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6
Using a 1/4-cup measuring cup, scoop up the celery root mixture, tightly pack it, and carefully drop it into the pan.
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7
Repeat until you have 4 mounds.
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8
Using a spatula, gently press each mound into a flat disk about 1/2 inch thick.
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9
Fry undisturbed until the bottoms of the latkes are golden brown, about 1 1/2 to 2 minutes.
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10
Flip and fry until the latkes are golden brown on the other side, about 1 1/2 to 2 minutes more.
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11
Transfer the latkes to the rack on the prepared baking sheet, season with salt, and place in the oven to keep warm.
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12
Using a slotted spoon, skim and discard any bits of latke remaining in the pan.
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13
Repeat frying the remaining celery root mixture 4 latkes at a time until all of the mixture has been used.
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14
(Make sure to skim the remaining bits of latke between each batch.)
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15
Place the mayonnaise and mustard or horseradish in a small bowl, season with pepper, and stir to combine.
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16
Spread about 1 teaspoon of the mayonnaise mixture on each latke.
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17
Divide the pastrami among the latkes.
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18
Top with the pickle slices and serve immediately.
