Add butter to your skillet or large pan, then add onions and fry for a few minutes - don't colour too much.
Sprinkle over flour and cook for a minute then add about 3/4 cup of milk and whisk until sauce thickens, then gradually add remaining milk, whisking continuously until sauce is thick and smooth.
Add the cheese and pesto, stir until combined then, add the finely chopped cauliflower and tuna.
Stir gently until all combined. Then add pasta.
Pour into a baking dish, top with bread crumbs and extra cheese and bake for about 20 mins until golden.