Heat the butter and oil in a pot. Add the flour and braise it on low heat, so it doesn't burn, while stirring nonstop.
Once it changes its color, dilute it with a little cooled sauce from roast peppers or water. Salt to taste, stir with a whisk until homogeneous and leave it on the stove for 1 min. to bubble.
Take the pot off the stove, add the yoghurt with the egg yolk beaten in it and return to the stove for another 1 min., add the lemon juice and your sauce is ready.
You can serve it with stuffed peppers, fried potato meatballs or cabbage leaf rolls!