Heat the oil in a large skillet over medium-high heat. Add the garlic and mushrooms and cook for about 4 minutes, until softened and lightly browned. Stir in the chard and heat until wilted, 1-2 minutes more. Remove from heat and season with salt and pepper to taste.
Use a slotted spoon to transfer the mushrooms and chard to a large bowl; discard any leftover liquid. Stir in the ricotta, Parmesan cheese, and Italian seasoning. Taste the filling and add more salt and pepper if desired, then fold in the egg.
Spread ¾ cup of marinara sauce on the bottom of a medium baking dish that's been sprayed with an oil mister (or cooking spray if you don't have a mister). Fill the pasta shells with the ricotta mixture and place them in the baking dish. Once all the shells are filled, spoon the remaining marinara sauce over the top, then sprinkle with the mozzarella cheese.
Cover the baking dish with foil and bake for 30 minutes; remove the foil and bake for 15 minutes more, or until the cheese is melted and the sauce is bubbling. Let the shells sit for 5 minutes before serving.
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