Heat olive oil in a large skillet over medium heat and cook onion and garlic until soft and translucent, about 5 minutes. Add chard stems and cook until softened, about 5 minutes.
Place chard leaves in pot, cover, and simmer over low heat until leaves are wilted, about 8 minutes. Season with vegetable stock, salt, and pepper; stir in tomatoes and feta cheese and cook until feta starts to melt, 2 minutes. Pour in cream and heat for another 2 minutes.