To brine the chicken, whisk the salt into 1 litre of cold water in a large bowl until the salt has totally dissolved, then add the herbs, garlic and the chicken. Cover and put in the fridge for at least 5 hrs (up to 24 hrs is fine).
Meanwhile, to make the butter, crush the garlic with a little salt, using the side of your knife to form a paste, then mix it into the softened butter with the herbs and lemon zest. Using a layer of cling film, roll the butter into a sausage shape and put in the fridge or freezer for at least 1 hr to firm up. This can be done up to two days ahead.
Remove the chicken from the brine but don’t rinse it, and instead pat dry. Using a sharp knife, make a small horizontal incision in the fattest part of each breast. Using your fingers, open up the incision to form a decent-sized cavity without cutting all the way through. Divide the butter mixture between the breasts and stuff it inside, leaving the chicken nice and plump, without any butter on show.
To make the coating, put the flour, eggs and panko crumbs in separate shallow bowls. Dip the chicken in the flour first, then in the egg and finally in the breadcrumbs. Repeat the egg and crumb process again so the chicken has a thick coating. This can be done the day before and kept in the fridge.
Heat oven to 220C/200C fan/gas 8. Place a heavy frying pan on a medium to high heat. Add a generous layer of vegetable oil and fry the Kievs individually until golden all over, working in batches if you need to. Place on a rack with a baking tray underneath and bake in the oven for 25 mins until deep golden and cooked through. Serve with sweet braised lettuce and peas.