1.Make cucumber salad: Trim ends from cucumber, then peel and halve lengthwise. Scoop out seeds; thinly slice crosswise into half-moons. Finely chop 1 tablespoon onion (save rest). In a medium bowl, whisk to combine chopped onions, 2 teaspoons vinegar, ¼ teaspoon sugar, and a pinch of salt. Whisk in 1 tablespoon oil. Add cucumbers, stirring to coat. Let stand at room temperature until step 5.
2.Pound and season chicken: Place chicken between 2 sheets of plastic wrap. Using a meat mallet (or heavy skillet), pound each piece to an even ⅛-inch thickness. Season all over with salt and pepper.
3.Bread chicken: Dust each piece of chicken with flour. In a shallow bowl, beat 1 large egg with a pinch each of salt and pepper. Transfer panko to a second shallow bowl. Dip chicken into egg, letting excess drip back into bowl. Press chicken into panko, flipping to coat all over and pressing to help breading adhere; tap off any excess.
4.Shallow fry chicken: Heat ¼-inch oil in a large heavy skillet (preferably cast-iron) over medium-high. Once oil is hot (it should sizzle vigorously when a pinch of panko is added), add chicken and cook, turning once, until golden, crisp all over, and cooked through (reduce heat if browning too quickly), 3–5 minutes per side. Transfer to a paper towel-lined plate. Season with salt.
5.Make mustard sauce: In a small bowl, whisk to combine Dijon, all of the sour cream, 1 teaspoon oil, and 2 teaspoons water. Season to taste with salt and pepper. Serve chicken schnitzel with cucumber salad and tangy mustard sauce on the side for dipping. Enjoy!
6.How to: Shallow Fry! We have some quick tricks for the perfect crunch. Make sure your oil is hot: it should reach 176°C with an oil thermometer. If you drop a pinch of panko into the oil, it sizzles vigorously. When working in batches, always make sure your oil comes back up to temperature between batches (if it’s browning too quickly, reduce the heat). If too low, the chicken will turn out oily instead of crispy.