Thinly slice the pork tenderloin into 1/2 inch medallions. Pound to 1/4 inch and make wider. Season with kosher salt and pepper, dust with flour on both sides and set aside on a plate.
In a skillet melt 2 tablespoons of butter and the olive oil. Sauté on each side until golden brown and edges are crispy. Remove medallions from pan and keep warm.
Add the garlic to the skillet and cook for a minute. Add the other 2 tbsp of flour to the pan and stir, cook for a minute. Whisk in the chicken stock and scrape up the browned bits. Sauce will thicken.
Add the zest from the lemon and chopped rosemary over the sauce, add the juice from the lemon and stir to incorporate. Cook on low for more 2 minutes.
Turn off the heat and add the remaining butter, stir and pour over the pork medallions and serve.