Preheat the oven to 400 degrees F. Grease a 13 by 9-inch glass baking dish liberally with butter.
Bring a large pot of salted water to a boil over high heat. Add the pasta and cook until tender but still firm to the bite, stirring occasionally, about 6 to 8 minutes. Drain the pasta and rinse with cold water. Set aside.
Using kitchen scissors or a knife, remove the leaves from the stems of the chard. Discard the stems. Chop the leaves into 1-inch pieces. In a large nonstick skillet, heat the oil over medium-high heat. Add the onion and cook until soft, about 5 to 7 minutes. Add the garlic and cook for 1 minute longer. Add the chard and cook, stirring constantly, until wilted, about 2 minutes. Allow the mixture to cool slightly. Place the ricotta cheese, peas, mozzarella cheese, Parmesan cheese, basil, salt, and pepper in the bowl of a food processor. Add the cooled chard mixture and blend until smooth. Spoon the mixture into the prepared pastry bag and fill each manicotti shell with the filling. Place the stuffed manicotti in the prepared baking dish.
In a medium heavy-bottomed saucepan, bring milk and cream to a simmer, over medium heat. Reduce heat to low. Add the fontina cheese and cook, stirring constantly, until the cheese is melted and the mixture is smooth. Remove the pan from the heat. Stir in the Parmesan cheese and basil.
Pour the sauce over the stuffed manicotti and sprinkle with the mozzarella cheese. Bake for 30 to 35 minutes until the top is golden. Cool for 5 minutes before serving.