Preheat oven to 350 degrees F.
In a small saucepan, heat the 2 tablespoons of canola oil over medium heat. Sprinkle in the flour and stir constantly to form a paste (you are making a roux, used to thicken the broth). Once a paste forms, slowly pour in the mushroom broth while whisking constantly to prevent lumps. Continue cooking until broth is thick. Meanwhile, layer green beans, mushrooms, and pearl onions in a casserole dish. Pour the thickened sauce over and bake for 25 minutes.
While casserole is baking, place leeks in a small saucepan and add just enough canola oil to cover. Place the saucepan over low heat and allow leaks to slowly crisp. Cook until the leeks have stopped making noise. Remove from oil and drain on paper towels.
Remove casserole from the oven, stir it, and top with the crispy leeks. Return it to the oven and bake for another 5 minutes. Serve.