In a large pot, cook the pasta in well-salted water until 2 minutes before al dente. Drain—reserving 1 cup of the cooking water—and set aside.
In a pan over medium-high heat, add the oil, garlic, leek and mushrooms and cook until just tender—about 5 minutes. Add the arugula, wine, salt and pepper and cook until the greens slightly wilt. Butter a casserole tin. Toss the penne with the vegetable mix, add the cheeses, mix well, put into the casserole tin and set aside.
Make the topping by toasting the breadcrumbs with the butter in a hot skillet until lightly brown and crunchy. Remove from the heat, transfer to a bowl to cool, then add the cheese to the breadcrumbs and set aside.
To make the sauce, melt the butter in a saucepan over medium-high heat. Add the flour and cook, stirring often, until lightly browned and nutty smelling—about 10 minutes. Add the milk to the butter/flour mixture—whisking constantly to combine. Add the nutmeg and allow the sauce to simmer for 8 minutes. Add some of the reserved pasta water until the sauce resembles a chowder consistency.
Remove from the heat and preheat the oven to 350°. Pour the sauce over the pasta/vegetable/cheese mixture, combine with a spoon, cover with foil and bake for 40 minutes. Remove the foil, top with the breadcrumb topping and bake for an additional 10 minutes.