Lightly pound the chicken thighs 1/2 inch thick and season with salt.
Put the flour, eggs, and panko breadcrumbs into 3 separate dishes and season each one with salt.
Dredge 1 piece of chicken in the flour, tapping off the excess. Dip in the egg, allowing the extra to drip off. Then coat the chicken with the panko, pressing to help it stick. Transfer the chicken to a baking sheet lined with wax paper. Repeat with the rest of the chicken.
Divide the oil between 2 large skillets and heat. Add the chicken and fry over medium-high heat, turning once, until golden brown and crispy, (about 3 minutes per side).
Allow them to drain on paper towels. Serve with tonkatsu sauce, mustard, and steamed rice.