In a large saucepot or Dutch oven, melt the butter and sauté the chopped onions until soft and translucent, about 5 minutes. Add the garlic and cook for 1 minute.
Add the chorizo to the onions and cook until the sausage loses its raw color, about 5 minutes.
Add the tomato paste and red pepper flakes, and then turn up the heat to high and cook for a minute.
Lower the heat to medium, add the prepped and drained mussels. Fold the mussels into the cooked chorizo.
Add the white wine and allow the mixture to bubble for a minute.
Cover to steam the mussels and the mussels open, 4-5 minutes.
Sprinkle the grated Gruyere cheese over the top of the cooked mussels. Cover with lid for one minute or until cheese is melted.
Serve immediately and garnish with chopped parsley and red pepper flakes if desired.