Melt 2 tablespoons butter in same skillet, add onions and season lightly with salt and pepper. Cook, covered, for 10 minutes, stirring occasionally, until onions soften. Then uncover and continue cooking, stirring often, until onions turn light golden brown, about 10 to 15 minutes longer. Add a splash of water every so often if bottom of pan gets too dark. Add garlic, thyme, wine, and chard. Cook until garlic is fragrant and chard is wilted, about 3 minutes. Sprinkle in flour and cook, stirring, for 1 minute longer, then stir in chicken stock and milk. Bring to a simmer and cook until mixture is thick enough to coat the back of a spoon. Turn off heat, taste it for seasoning and add salt and pepper, to taste. Return chicken to skillet, add scallions, and mix well. Dot top with crumbled goat cheese.