Finely chop the onion. Heat oil in a saucepan over medium heat, then sauté the onion for 2-3 minutes until softened.
Add the sour cream, pressed garlic, frozen herbs, dried tarragon, and mustard. Bring to a boil over medium heat, then reduce to medium-low heat and simmer for 5 minutes. Remove from heat and let cool for 3-5 minutes.
Place each schnitzel between two sheets of plastic wrap and tenderize by striking gently with the bottom of a pan. Season both sides with salt and pepper.
Set up three shallow bowls: one with flour, one with beaten egg, one with breadcrumbs. Coat each schnitzel first in flour (shake off excess), then in egg, then in breadcrumbs.
Heat a generous amount of oil or fat in a large pan over medium-high heat. Once hot, place the schnitzel in the pan and fry for about 3-4 minutes per side until golden brown, checking that the internal temperature reaches 63°C (145°F).
Transfer the schnitzel to plates and pour the sour cream sauce over the top.