Start by rinsing the quinoa under cold water. In a medium saucepan, combine the quinoa, vegetable broth, and a pinch of salt. Bring it to a boil, then reduce the heat to low, cover, and let it simmer for about 15 minutes or until the quinoa is fluffy and the liquid is absorbed. Set it aside to cool slightly.
In a large frying pan or cast-iron skillet, heat the olive oil over medium heat. Slice the yellow onion thinly and add it to the pan. Sauté for about 5 minutes until the onions become translucent.
Pour in the balsamic vinegar and a pinch of sea salt. Continue to cook the onions for another 5-7 minutes, allowing them to caramelize and develop a deep, rich flavor. Stir occasionally to prevent sticking.
While the onions are cooking, wash and shred the chard leaves. Once the onions are caramelized, add the chard to the pan and cook for about 3-4 minutes until wilted.
In a mixing bowl, whisk together the eggs, heavy cream, salt, and black pepper until well combined.
Add the cooked quinoa to the pan with the chard and onions. Pour the egg mixture over the top, ensuring everything is evenly distributed.
Reduce the heat to low and let the frittata cook for about 10-12 minutes, or until the edges are set but the center is still slightly jiggly. You can cover the pan with a lid to help it cook evenly.
Preheat your broiler. Once the frittata is almost set, place the skillet under the broiler for 2-3 minutes, just until the top is golden and puffed. Keep a close eye to avoid burning.
Remove the skillet from the oven and let the frittata cool for a few minutes. Slice into wedges and serve warm or at room temperature.
