Rinse the quinoa under cold water using a fine mesh sieve to remove the saponin coating.
In a medium saucepan, combine rinsed quinoa, vegetable broth, and a pinch of salt. Bring to a boil, then reduce heat to low, cover, and simmer for about 15 minutes until fluffy and liquid is absorbed. Remove from heat and set aside.
While quinoa cooks, slice the yellow onion thinly.
In a large skillet, heat olive oil over medium heat. Add sliced onion and a pinch of sea salt. Cook for 15-20 minutes, stirring occasionally, until caramelized and golden brown.
Add balsamic vinegar to the onions, stir to coat, and cook for an additional 2-3 minutes until vinegar is reduced and sticky. Remove onions from skillet and set aside.
In the same skillet, add shredded chard and sauté for 5-7 minutes until wilted and tender.
Remove from heat and mix in the cooked quinoa and balsamic onions until well combined.
In a mixing bowl, whisk together eggs, heavy cream, salt, and black pepper until well combined.
Preheat oven to 375°F (190°C). Pour the egg mixture over the quinoa-chard-onion mixture in the skillet, gently stirring to combine.
Cook on stovetop for 3-4 minutes until edges begin to set.
Transfer skillet to preheated oven and bake for 20-25 minutes until frittata is set and lightly golden on top.
Remove the frittata from oven and let cool for a few minutes. Slice into wedges and serve warm.