2In a large, deep, ovenproof frying pan over medium heat, warm a glug of olive oil, then add the onion, sliced chard ribs, and a big pinch of salt. Cook, stirring, until starting to turn golden, about 5 minutes. Add the mushrooms and chard leaves and cook, stirring, until tender, about 5 minutes. Add the crushed tomatoes and bring to a simmer. Lower the heat to medium-low and simmer for 10 minutes while you boil the pasta.
3While the sauce is simmering, bring a large pot of salted water to a boil over high heat. Add the tortellini and cook according to the package directions until just shy of al dente. Drain well.
4Add the tortellini to the sauce and stir to combine well. Transfer to a 9×13 inch glass baking dish. Top with the mozzarella and parmesan. Bake until bubbly and the cheese is melted and golden, about 20 minutes. Serve garnished with the basil, if you like.