Prepare the Filling: Preheat your oven to 375°F / 190°C.
Heat the olive oil in a large pot over high heat.
Add the chopped chard stems and onion, cooking and stirring often for about 8 minutes until softened.
Stir in the chard leaves and minced garlic.
Cover the pot, reduce the heat to medium-low, and cook for 5 minutes until the leaves are tender.
Season generously with salt and pepper.
Assemble the First Layer: In a bowl, combine 2½ cups of the tomato sauce with the chopped fresh basil.
Spread ½ cup of this tomato-basil sauce evenly over the bottom of a 10-inch springform pan.
Arrange 3 corn tortillas in a single, slightly overlapping layer over the sauce.
Build the Casserole: Season the ricotta cheese with salt and pepper.
Spread 1 cup (240 ml) of the seasoned ricotta over the tortillas.
Top with ½ cup of tomato-basil sauce, half of the cooked Swiss chard mixture, ⅓ cup of mozzarella, and 1 Tbs. of Romano cheese.
Repeat this entire layer once more: 3 tortillas, the remaining 1 cup (240 ml) of ricotta, ½ cup tomato-basil sauce, the rest of the chard mixture, another ⅓ cup mozzarella, and 1 Tbs. Romano.
Final Layer and Bake: Place the last 3 tortillas on top.
Spread the remaining 1 cup (240 ml) of tomato-basil sauce over the tortillas.
Cover the pan tightly with foil and bake for 45 minutes.
Brown and Serve: Remove the foil and sprinkle the top with the remaining ⅓ cup mozzarella and 1 Tbs. Romano cheese.
Bake uncovered for another 10 minutes, or until the cheese is melted and bubbly.
Let the casserole stand for 5 minutes before carefully removing the springform ring.
Cut into wedges and serve immediately with the remaining warmed tomato sauce on the side.
Don't have a springform pan? A deep 9-inch round cake pan or an 8x8-inch square baking dish works well too.
Grease it thoroughly for easier serving.