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1
Bring a small pot of salted water to a boil.
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2
Add the chopped carrot and celery root and cook over high heat until just tender, 4 to 5 minutes.
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3
Drain and let cool under running water; drain again.
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4
Transfer the vegetables to a plate lined with paper towels and pat dry.
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5
Melt the butter in a large saucepan.
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6
Add the onion and cook over moderate heat until softened, about 5 minutes.
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7
Add 2/3 cup of the flour and cook, stirring, for 1 minute.
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8
Transfer the paste to a large heatproof bowl and slowly whisk in the milk to form a smooth slurry.
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9
Return the mixture to the saucepan and cook over moderate heat, stirring, until bubbling and very thick, about 6 minutes.
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10
Add the tuna to the white sauce along with the carrots, celery root, salt and pepper.
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11
Scrape the croquette mixture into a lightly oiled medium bowl and refrigerate until chilled and firm, at least 4 hours, or preferably overnight.
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12
Spread the remaining 1 1/3 cups of flour in a shallow bowl or pie plate.
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13
Put the eggs and bread crumbs in 2 shallow bowls or pie plates.
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14
Using 2 spoons, shape slightly rounded tablespoons of the croquette mixture into oval patties and drop them, 3 or 4 at a time, into the flour.
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15
Turn the croquettes to gently coat them with flour, then dip them in the eggs and coat with bread crumbs.
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16
Return the croquettes to the eggs and then the bread crumbs to give them a light second coating.
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17
Transfer the finished croquettes to a baking sheet lined with wax paper and continue shaping and coating the remaining croquettes.
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18
Loosely cover with wax paper and refrigerate until chilled, at least 30 minutes.
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19
Heat 1 inch of vegetable oil in a large deep skillet until shimmering.
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20
Working in batches, cook the croquettes over moderately high heat, until golden and crisp, turning frequently, about 2 minutes; lower the heat if they brown too quickly.
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21
Using a slotted spoon, transfer the croquettes to paper towels to drain.
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22
Serve warm or at room temperature.
