Cook the pasta according to pack instructions, slightly al dente, drain and rinse in the sink under cold water to stop the pasta cooking and sticking together.
Chop the greens only of the scallions and leeks, finely chop the mushrooms and cut the broccoli into small bite-sized florets
While the pasta is cooking, cook the veg and start the bechamel.
For the bechamel in a non stick pot heat the olive oil once hot, add the rice flour into the oil and whisk for 2 minutes stirring continuously until golden, add the oat milk slowly to the roux (the flour and oil mixture) whisking continuously. Then add the seasoning and remaining ingredients. Bring it to the boil and reduce to a simmer being careful that nothing sticks to the bottom, whisking often, once it has reached the desired degree of thickness, remove the Bay leaf from the sauce, remove from the heat, taste and season if needed.
For the veg, heat 1 tbsp of oil in a large non stick pan over a high heat, add the scallion and leek greens with a good pinch of salt , reduce to medium and cook for 3-4 minutes until the greens are nicely softened, add the mushrooms and cook for 5 minutes until the mushrooms are reduced, add the broccoli florets and water, bring to a simmer then reduce heat, cover tightly and steam until broccoli is just cooked 5-8 minutes
Mix the pasta, bechamel and cooked veg mixture, transfer to an oven proof dish, smoothing out the surface slightly
Mix all the topping ingredients together well and scatter over the pasta bake. Pop in the oven for 20 minutes or until the sauce is bubbling and the crumb top is golden brown.