Bring a large pot of salted water to a boil. Add the Swiss chard and cook until just wilted, about 2 minutes.
Drain and finely chop the chard, then use a potato ricer on the smallest setting or a kitchen towel to extract as much water as possible. Set aside.
Heat the oil in a large skillet over medium heat. Once hot, add the onion and sauté until soft, about 5 minutes.
Add the sausage and sauté, breaking into smaller pieces as it cooks, until browned and cooked through, about 8 minutes. Remove from heat and allow to cool.
Preheat the oven to 325°F. Spray an 8- x 8-inch baking dish with cooking spray.
In a large bowl, whisk together the eggs, egg substitute, half and half, and salt and pepper.
Stir in the chard and sausage mixture, then add the feta. Mix gently until well-combined, then pour the mixture into the prepared baking dish.
Bake until the frittata is set in the center, about 50 minutes.
Transfer the baking dish to a rack and allow it to cool for 15 to 20 minutes.