Rinse mussels under cold water and remove any beards. Discard any open or damaged shells.
In a large pot, heat olive oil over medium-high heat. Add onions and garlic and cook until softened (about 3 minutes).
Pour in clam juice/fish stock, white wine, thyme leaves, bring to a boil then reduce the heat.
Cover with a lid or foil loosely for even steaming. Cook mussels over low heat until they open (about 5 minutes). Discard any that do not open.