For the pasta: Bring a large pot of salted water to a boil. Add the penne and cook until al dente according to the package instructions. Drain and set aside.
For the pureed greens: Meanwhile, add the spinach, arugula, basil, mint, parsley, red pepper flakes, salt, pepper and garlic to a food processor. With the machine running, drizzle in the olive oil and puree. Stop before the mixture is completely smooth; you want a bit of texture. Set aside.
For the cheese sauce: Melt the butter in a large pot over medium heat. Sprinkle in the flour and mustard powder, whisking to combine. Cook, whisking constantly, for a minute or so. Pour in the milk, whisking constantly. Cook the sauce until thick and bubbly, 3 to 5 minutes. Whisk in the half-and-half, salt and pepper. Add the Cheddar, fontina and Monterey Jack. Stir until the cheese all melts.
For the assembly: Butter a 9-by-13-inch baking pan.
Combine the melted butter, breadcrumbs and parsley in a small bowl.
Add the cooked penne to the cheese sauce, making sure every noodle is covered in sauce. Next, stir in the pureed greens until barely combined. Pour the pasta mixture into the buttered baking pan and top evenly with the breadcrumbs. Bake until bubbly and golden brown, 20 to 25 minutes.
If you are serving later, top the assembled casserole with aluminum foil and pop in the freezer. When you are ready to serve, allow it to thaw in the fridge overnight and bake the following day at 350 degrees F until golden brown and cooked through, about 1 hour covered and an additional 15 minutes uncovered.