Make the dough: sprinkle the yeast and sugar over the water in a large bowl and let sit 10 minutes. Add the oil, salt, and half the flour. Beat smooth. Knead in the remaining flour, more as needed, to create a soft dough that doesn’t stick to the hands. Cover in a bowl and let rise until doubled, 45 minutes. Punch down the dough and stretch it out to fit on a pizza sheet.
While the dough is rising, make the béchamel: Bring the milk to the boiling point in a saucepan with the bay leaf and onion. Turn off the heat, cover, and leave to infuse 15 minutes. Melt the butter in another saucepan, then whisk in the flour, and keep whisking on the heat for a full minute to cook out the flour taste. Slowly whisk in the milk, until smooth, and cook, stirring until thick, 3 minutes. Stir in the crab, and half the cheese.
Heat the oven to 400ºF/200ºC. Distribute pieces of hot-smoked salmon over the pizza crust. Spoon the sauce over top (this will protect it from overcooking). Sprinkle with the remaining cheese and the chopped white part of the green onions, reserving the chopped green for garnish later. Bake 25 minutes until the crust is crisp. Scatter the green of the onions over. (If you like, lay on a few slivers of cold-smoked salmon for extra colour) Serve.