How to Make It
In a large skillet, heat the olive oil with 1 teaspoon of the truffle oil. Add the mushrooms, season with salt and pepper and cook over moderate heat, stirring occasionally, until golden and tender, 5 to 7 minutes. Transfer to a plate. Finely chop the mushrooms and transfer to a small bowl. Stir in the remaining 1/2 teaspoon of truffle oil.
In a small saucepan, bring the milk to a simmer. In a medium saucepan, melt the butter. Add 1/2 cup plus 1 tablespoon of the flour and whisk until smooth. Cook over low heat, whisking frequently, for 5 minutes. While whisking constantly, slowly drizzle in the hot milk. Cook over low heat, whisking frequently, until the béchamel is very thick and smooth, about 20 minutes. Remove from the heat and fold in the mushrooms and cheese. Season with salt and pepper. Transfer the mixture to a bowl and press plastic wrap directly onto the surface. Refrigerate until firm, about 1 hour.
Spread the remaining 1/2 cup of flour in a shallow bowl. In another shallow bowl, whisk the eggs. Spread the breadcrumbs in a third shallow bowl. Roll the mushroom mixture into eighteen 1 1/2-inch balls. Dredge them in the flour, then coat with the egg and dredge in the breadcrumbs. Transfer to a plate.
In an enameled medium cast-iron casserole, heat 2 inches of canola oil to 350°. Line a large plate with paper towels. Working in 2 batches, fry the croquettes until golden brown, about 4 minutes. Transfer to the paper
towels to drain. Arrange on a platter and sprinkle with truffle salt. Serve with lemon wedges.