Place the pieces of chicken next to each other on a cutting board and cover with cling wrap/film over the top. Gently pound the chicken just enough so that they are relatively even in thickness. Remove the cling wrap/film.
Season both sides of the chicken with pepper. If you like you can also add a little salt, but bear in mind prosciutto is a little salty - I'd suggest only on first side if at all. Finely chop one of the sage leaves and press this onto the chicken on the second side.
Top each of the pieces of chicken with a slice of prosciutto so that it covers the chicken as far as possible (you can fold over a little, if needed, where the chicken narrows). Top each piece with a large sage leaf then use a cocktail stick through the sage leaf, prosciutto and chicken to secure it all in place but lie flat (see photos).
Warm the oil in a large skillet/frying pan or braiser over a medium-high heat. Add the chicken pieces prosciutto side down and cook for 1 - 2 minutes to lightly crisp the prosciutto and start the sage releasing its aroma.
Turn the chicken over, taking care not to break up the prosciutto if it sticks slightly, then cook for another 3 - 5 minutes until the chicken is cooked through. Remove the chicken and set aside, covering with foil to keep them warm.
Meanwhile, add the butter to the same skillet and once it is relatively well melted, add the flour. Cook for a minute to let it sizzle and brown slightly.
Then, deglaze the pan with the wine followed by the stock (add the stock just around half a minute after, to let the wine boil off a little of the alcohol). Continue to simmer the sauce for about 2 - 3 minutes, stirring regularly to both scrape any browning from the bottom of the skillet and avoid it becoming lumpy. It should thicken slightly. Add a little black pepper and salt, if needed to taste. Then add the lemon juice, stir through and serve the chicken with the sauce poured over the top.