Preheat the oven to 190 degrees C. Melt the butter in a small saucepan, add the flour and stir for several seconds, then gradually pour in the milk. Bring to the boil, season with salt and pepper, and cook for 2 minutes.
Remove the pan from the heat. Stir in 1 1/2 tablespoons of lemon juice, then set the white sauce aside.
Cut the avocados in half;remove the stones, peel, then slice the flesh thinly along their length. Sprinkle lightly with the remaining lemon juice to prevent browning.
Lay the tortillas out flat on a work surface. Put some salmon and avocado slices on each one. Roll them up and place them, side by side, in a shallow ovenproof dish, then pour on the white sauce. Bake for 20 minutes until golden. Garnish with rosemary before serving.