Preheat the oven to 180°C fan. Bring a large saucepan of salted water to the boil. Add the oil and lasagna sheets and boil for 6 minutes, until cooked but al dente. Drain well and spread the sheets out on a large, lightly oiled baking tray and drizzle with more oil to prevent them from sticking. Put all the ingredients for the béchamel in a large bowl with salt and pepper. Whisk until smooth, then pour half of this mixture into a baking dish. Lay out a sheet of lasagna on a clean work surface with the shorter side facing you. Spread a spoonful of pesto over the surface of lasagna. Lay three asparagus spears on top, horizontally, then roll the whole thing into a cannelloni shape. Carefully transfer the cannelloni, seam side down, into the dish of béchamel. Continue with the remaining cannelloni. Pour the remaining béchamel over the pasta and spoon over another tablespoon of oil. bake for 35 minutes and leave to settle for 10 minutes after you remove it from the oven. Serve warm with more pesto if you wish.