Slice each chicken breast horizontally to create thin cutlets (about 1/4-inch thick). If pieces are uneven, place between plastic wrap and pound gently with a meat mallet to an even 1/4-inch thickness. Pat dry with paper towels.

Season

Season both sides of the flattened chicken cutlets with kosher salt, black pepper, garlic powder, and paprika.

Set up breading stations

Place the flour in a shallow dish. In a second shallow bowl, whisk the eggs with the milk until combined. In a third shallow dish, combine the breadcrumbs and grated Parmesan.

Bread the cutlets

Working one at a time, dredge each cutlet in flour, shaking off excess, dip into the egg mixture, then press into the breadcrumb-Parmesan mixture until evenly coated. Set on a wire rack or plate and let rest 5 minutes so coating adheres.

Fry

Heat a large skillet over medium-high heat and add vegetable oil and optional butter. When oil is hot (a breadcrumb sizzles on contact), add cutlets without crowding. Fry until golden brown and cooked through, about 3 minutes per side for thin cutlets (internal temperature 165°F / 74°C). Transfer to a wire rack or paper towel-lined plate to drain. Keep warm in a low oven (200°F / 95°C) if needed.

Chimichurri (optional)

Combine the chopped parsley, cilantro (if using), minced garlic, red wine vinegar, olive oil, red pepper flakes, salt, and pepper in a small bowl. Stir until blended. Taste and adjust seasoning; let sit at room temperature at least 10 minutes to allow flavors to meld.

Sandwich assembly / plating

For sandwich

Slice the baguette lengthwise (if desired, toast lightly). Spread mayonnaise or aioli on the cut sides. Place a lettuce layer on the bottom, add one or two milanesa cutlets (trim to fit if necessary), top with tomato slices and pickled red onion, and finish with a spoonful of chimichurri or drizzle. Close, press gently, and cut into portions.

For plated option

Place one or two milanesa cutlets on a plate alongside mixed salad greens tossed with olive oil, lemon juice or vinegar, and salt and pepper to taste. Spoon chimichurri over the cutlet or serve on the side.

Serve

Serve immediately while the milanesa is hot and crisp. Leftovers can be refrigerated up to 3 days; reheat briefly in a 375°F (190°C) oven to restore crispness.